Portal to the Deep
The deep sea covers over 60% of our planet, yet less than 1% of it has been explored. [1] Dive in to learn more about Guam’s unique deep-sea ecosystems.
The Sunlit Zone
The sunlit zone is the sunny, shallow, top layer of the ocean where most marine species live. This zone reaches down to 656 ft deep, which is the deepest point where photosynthesis can still occur.
0-656 ft deep (0 - 200 m) [2]
Lots of sunlight, warm water, and abundant food
A Pacific double-saddle butterflyfish (Chaetodon ulietensis) on the reef in Piti.
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The sunlit zone is home to plants, algae, phytoplankton (plant-like plankton), and most corals since these species rely on light for photosynthesis.
The abundance of phytoplankton in the sunlit zone is particularly important because not only do they produce oxygen as a result of photosynthesis, but they also support the entire food web. [3]
Here’s how: phytoplankton are an important food source for zooplankton, which are tiny, free-floating animal plankton. Zooplankton, like krill and small shrimp, feed larger organisms like plankton-eating fish, filter-feeding clams, and even large animals like blue whales.
Since there are so many species living in the sunlit zone, each creature must find its own unique way to adapt and survive.
Some blend in using countershading: being dark on top to blend in with the dark ocean below, and a light underside to blend in with the sunlit ocean. This way, they go undetected by predators and prey looking at them from above and from below.
Pelagic, or open ocean, creatures like sharks and dolphins typically use countershading.
Some are bright, vivid colors or patterns to signal to predators that they are poisonous or venomous.
Some try to blend in with their surroundings, whether that be the tan-colored sand, colorful coral, brown rocky substrate, or other shallow habitats.
The Twilight Zone
The twilight zone is the dimly lit midwater zone of the ocean and is considered to be where the deep sea begins. The bottom of the twilight zone is the deepest point that light can penetrate.
656-3,300 ft deep (200-1,000 m) [2]
Dim sunlight, near-freezing water, and scarce food
A deep-sea swimming balate’! Photo: 2016 NOAA Mariana Trench expedition.
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Once you get into the twilight zone, the water is on average 39 degrees F (4 degrees C), which is only a few degrees warmer than freezing! [4] On top of that, the light is very dim and the pressure from the mass of water above is increasingly strong.
Few creatures can tolerate these harsh conditions, but the ones that can have very unique adaptations that allow them to survive. For example:
Some have huge eyes for seeing in dim light!
Since the twilight zone is mostly the open ocean with no seafloor to rest on, many species are gelatinous and lightweight and drift through the open seas, like jellies and siphonophores.
Most creatures in the twilight zone use bioluminescence: the ability some species have to produce light through a chemical process in their body. Bioluminescence is used in many different ways:
To attract and catch prey, like the anglerfish, which dangles its bioluminescent lure right in front of its mouth to attract curious prey and snap them up if they get too close
To attract mates.
To see their surroundings.
To scare off predators, like the firefly squid, which squirts out bioluminescent ink to stun and ward off predators.
To blend in! This is called counter-illumination: some species have bioluminescent spots on their underside, and when they glow, they blend in with the dimly sunlit surface. But for a predator looking down on them from above, their dark topside blends in with the darkness below.
The Midnight Zone
The midnight zone is the ocean’s pitch black depths, too deep for sunlight to reach. This zone accounts for 70% of the ocean, making it the largest habitat on Earth. [5]
3,300-13,100 ft deep (1,000-4,000 m) [2]
Zero sunlight, near-freezing water, and scarce food
A pricklefish (Malacosarcus sp) seen on the 2016 NOAA Mariana Trench expedition.
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With no sunlight, many creatures barely use their sense of sight and have heightened senses of smell, touch, taste, and hearing to get around. Some even have adapted unusual appendages to help them better sense their environment.
For example, the tripod fish has long tripod-like fins, which act as stilts and also act as sensitive antennae that can detect movement in the water.
Meals are few and far between, so many midnight zone species have extendable stomachs and huge mouths so that whenever food does drift down, they can eat the entire thing right then and there.
For example, the gulper eel can expand its jaw to engulf massive prey if needed.
Due to limited supply of nutrients, many animals rely on filter-feeding from currents, or eat tiny bits of decaying organic debris that slowly sinks from the surface waters known as “marine snow”. Other animals take advantage of “whale falls”: a rare occurrence in which a dead whale sinks all the way to the bottom of the ocean and becomes a meal for many deep-sea scavengers.
The Abyssal Zone
13,100-19,700 ft (4,000-6,000 m) [6]
The abyssal zone makes up the deeper depths beyond the midnight zone. Three-fourths of the ocean’s seafloor exists at these depths [6]
The Hadal Zone
19,700-36,070 ft (6,000-10,994 m) [6]
This zone only occurs in deep-sea trenches, and only a few parts of the world are home to trenches this deep. [7] Deep-sea trenches occur where two tectonic plates meet and one plate slides under the other.
The Mariana Trench
The deepest part of the ocean happens to be right in Guam’s backyard: the Mariana Trench.
At its deepest point, the Challenger Deep, the Mariana Trench reaches 35,876 ft (8.8 miles) below sea level. [8]
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This trench was created when the Pacific tectonic plate was pushed under the Philippine tectonic plate.
The trench is 50,532,102 acres, which is five times longer than the Grand Canyon. [9]
The Mariana Trench is deeper than Mount Everest is tall! [9]
The Challenger Deep was named after the HMS Challenger, which was the first oceanographic survey ship to chart its depth in 1875 [8]
Where is the Mariana Trench?
The Mariana Trench is a long, crescent-shaped trench that hugs the Mariana Archipelago, including Guam and the CNMI. On average, it is approximately 124 miles east of the Mariana Islands. [10]
Most of the Mariana Trench lies within the exclusive economic zones (EEZs) of the Commonwealth of the Northern Mariana Islands (CNMI) and Guam, and a small part of the trench, including the Challenger Deep, lies within the EEZ of the Federated States of Micronesia (FSM). [11]
Map of the U.S EEZ surrounding the Mariana Trench. Credit: Hourigan et al. 2021
Who manages the Mariana Trench?
For the part of the trench in Guam’s and CNMI’s waters, the Mariana Trench National Marine Monument was established through presidential proclamation in 2009 to protect the trench’s geographical features and unique deep-sea ecosystems. [12]
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The U.S. Fish and Wildlife Service and NOAA jointly manage the Mariana Trench National Monument, in collaboration with the CNMI government, the U.S. Department of Defense, the U.S. Department of State, the U.S. Coast Guard, and the Mariana Trench Monument Advisory Council. [9]
The Mariana Trench National Marine Monument spans 95,216 mi^2 and includes 3 main areas, or “Units”: the Island Unit, Trench Unit, and Volcanic Unit. The Trench Unit has been designated as the Mariana Trench National Wildlife Refuge, and the Volcanic Unit has been designated as the Mariana Arc of Fire National Wildlife Refuge. [12]
This Monument also includes the Sirena Deep, which is the second deepest point in the Mariana Trench, and the third deepest point in the entire ocean. [12]
Mariana Trench National Monument website: https://www.fws.gov/national-monument/mariana-trench-marine
Mariana Trench National Monument Poster created by NOAA fisheries. Access the full poster here
How has Guam been involved with the Mariana Trench?
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This trench was originally named “the HMRG Deep” after the group that discovered it in 1997, but was renamed “the Sirena Deep” after a local contest was held in 2009 for local Guam and CNMI students to submit their ideas for a new name. [13]
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She participated in a 2009 expedition with the Woods Hole Oceanographic Institute and a 2014 expedition with the Schmidt Ocean Institute. She has also participated in other deep-sea expeditions in other areas.
Woods Hole Oceanographic Institute 2009 Mariana Trench expedition: https://archives.whoi.edu/nereus/main/nereus/mariana-trench-2009.html
Schmidt Ocean Institute 2014 Mariana Trench expedition: https://schmidtocean.org/cruise/exploring-the-mariana-trench/
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While the trip was unfortunately cut short due to poor weather, [16] the team still managed to make a significant discovery. At 6.6 miles deep, these scientists recorded the deepest sighting of xenophyophores, which are some of the largest single-celled organisms on the planet. These xenophyophores are only found in deep-sea ecosystems, but previously had only been found 4.7 miles deep. [15]
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Learn more about the Aquarium of Guam, formerly known as Underwater World: https://www.aquariumofguam.com/
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The display includes a sample of deep-sea mud and volcanic rock extracted from the Mariana Trench on a recent expedition.
Learn more about the Guam National Wildlife Refuge: https://www.fws.gov/refuge/guam
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The winning names were: Paraliparis echongpachot, since echong pachot means “crooked mouth” in CHamoru, and the fish has a characteristically crooked mouth; and Paraliparis kadadakaleguak, since kadada’ kaleguak means short rib bones, which is a feature of this species. These new scientific names were officially published and accepted in 2021. [17]
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Aboard cruise NA171 in May 2025, exploring the Mariana volcanic arc:
Shannon Seleen, GDOE GATE teacher, served as a Science Communication Fellow.
Amanda Dedicatoria, freelance science communicator, served as the Lead Science Communication Fellow
Amanda was also a Science Communication Fellow aboard the E/V Nautilus’ expedition in Palau in 2024 and presented at the Guam museum in 2025 about this expedition and our connections to the deep sea.
Aboard cruise NA172 in June 2025, exploring mud volcanoes and the Mariana Trench:
Amber Pineda, UOG undergraduate, served as a ROV Engineering Intern
Shelterihna Alokoa, community educator, served as a Science Communication Fellow.
Linda Tatreau (front row, blue shirt) with the Schmidt Ocean Institute research team on their 2014 Mariana Trench expedition. Photo: Schmidt Ocean Institute
The preserved specimen of the deep-sea snailfish named by Guam students: Paraliparis kadadakaleguak. Photo: Brian Sidlauskas, Oregon State University
Deep Sea Exploration History in the Marianas
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1960 - the first expedition to the Challenger Deep with Jacques Piccard and Don Walsh aboard the bathyscaphe, Trieste
2012 - the second expedition to the Challenger Deep with James Cameron aboard the DEEPSEA CHALLENGER submersible
2019 - the third expedition to the Challenger Deep with Victor Vescovo aboard the deep submergence vehicle (DSV) Limiting Factor
2021 - first pacific islander to descend to the Challenger Deep, Micronesian Nicole Yamase, along with Victor Vescovo aboard the DSV Limiting Factor
2022 - recent expedition to the Challenger Deep with Dawn Wright and Victor Vescovo aboard the DSSV Limiting Factor
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2009 Mariana Trench expedition with Guam teacher Linda Tatreau on board: Woods Hole Oceanographic Institution - R/V Kilo Moana and hybrid remotely operated vehicle (HROV) Nereus
2014 Mariana Trench expedition with Guam teacher Linda Tatreau on board: Schmidt Ocean Institute - R/V Falkor
2016 Deepwater Exploration of the Mariana Islands: NOAA Office of Ocean Exploration and Research - NOAA Ship Okeanos Explorer
2025 Mariana Arc Volcanic Exploration (NA171): The Nautilus Live, Ocean Exploration Trust - E/V Nautilus
The remotely operated vehicle (ROV) Deep Discoverer being recovered after a dive on the 2016 NOAA Deepwater Exploration of the Marianas.
Hydrothermal vents
The Mariana Trench is lined with deep-sea volcanoes and other geological wonders like hydrothermal vents, mud volcanoes, and pools of molten sulfur! [18]
For example, the Eifuku submarine volcano looks like a series of underwater chimneys spewing extremely hot carbon dioxide measured at 217 degrees Fahrenheit. [18] This particular underwater chimney pictured in the background is called “Champagne vent”. Photo: Submarine Ring of Fire 2014 - Ironman, NOAA/PMEL, NSF
Check out the incredible hydrothermal vents of the Hafa Adai vent field. Video from the E/V Nautilus’ 2025 Mattingan: Mariana Arc Volcanic Exploration (NA171) expedition
Mussels and shrimp surrounding a vent in the Mariana Trench. Photo: Submarine Ring of Fire 2014 - Ironman, NOAA/PMEL, NSF
Snails living on a vent at the Chamorro seamount in the Mariana Trench. Photo: NOAA 2016 Exploration of the Marianas
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Some animals live by hydrothermal vents, which are like underwater volcanoes that release heat and minerals into the water.
Some bacteria in these hydrothermal vents can even do chemosynthesis, which is similar to photosynthesis but uses chemicals from these hydrothermal vents instead of sunlight to produce food.
Much like phytoplankton form the base of the food web in many shallow-water ecosystems, these chemosynthetic bacteria form the base of the food web in many hydrothermal vent ecosystems. These bacteria are preyed on by small shrimp called amphipods and copepods, which are then preyed on by larger crabs, snails, worms, and cephalopods. [13]
Deep Sea Reefs
The deep sea has coral reefs, too! There are actually three types of corals, depending on how deep they live:
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Live in waters less than 98 ft deep (30 m)
Adapted to live in warm, sunlit waters
Can only be found in tropical waters
Have photosynthetic symbiotic algae (zooxanthellae) and depend on them to produce food, but also catch some food with tentacles
Come in a range of shapes, including dense boulder-shaped colonies
[19]
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Where these reefs live depends on how clear the water is and how deep light can penetrate!
Some mesophotic reefs are 98-164 ft deep (30-50m [19]), while others in clear, tropical waters are 98-490 ft deep (30-150 m) [20]
Adapted to live in cooler, lower-lit waters [19]
Can be found in tropical and subtropical waters [20]
Have zooxanthellae and get some food from them, and also catch food with tentacles [19]
Usually grow in shapes that maximize light absorption [19]
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Depending on how deep light can penetrate, some deep-sea corals are found deeper than 164 ft [19], and others are found deeper than 490 ft (150+ m) [20]
Adapted to live in cold waters 39-54˚F down to 30˚F with little to no sunlight [19]
Can be found in waters anywhere in the world [21]
Do not have zooxanthellae and only catch food with tentacles [19]
They usually grow in shapes that maximize surface area in which to catch food particles drifting through the water, like fan, feather, and tree shapes. [21]
Check out this video of some incredible mesophotic reefs in the Marianas.
Video from the E/V Nautilus’ 2025 Mattingan: Mariana Arc Volcanic Exploration (NA171) expedition
In addition to the abundant shallow-water coral reefs surrounding the Mariana Islands, our region is also home to expansive deep-sea coral ecosystems. To date, over 100 species of deep-sea corals (50+ m deep) have been documented in the Mariana Trench and Mariana Arc of Fire National Wildlife Refuges within the Mariana Trench Marine National Monument. [22]
Deep-sea corals and sponges make a great habitat for deep-sea fish, crabs, brittle stars, worms, and other organisms. These organisms often are wrapped around or hanging onto coral branches while feeding in the current, or hiding inside their delicate and complex structures. Deep-sea corals make such a great habitat for other species that they are bustling with life and are considered hotspots for biodiversity in the deep sea! [23]
Inhabitants of the Mariana Trench
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Background photos from the NOAA Office of Ocean Exploration 2016 Deepwater Exploration of the Marianas, unless otherwise specified
Sources cited
[1]https://www.oceandiscoveryleague.org/about
[2]https://www.nsf.gov/science-matters/dive-research-worlds-ocean
[3]https://www.whoi.edu/know-your-ocean/ocean-topics/how-the-ocean-works/ocean-zones/sunlit-zone/
[5]https://www.whoi.edu/know-your-ocean/ocean-topics/how-the-ocean-works/ocean-zones/midnight-zone/
[6]https://www.noaa.gov/jetstream/ocean/layers-of-ocean
[7]https://www.whoi.edu/know-your-ocean/ocean-topics/how-the-ocean-works/ocean-zones/hadal-zone/
[8]https://www.britannica.com/place/Challenger-Deep
[9]https://www.fws.gov/national-monument/mariana-trench-marine
[10]https://deepseachallenge.com/the-expedition/the-mariana-trench/
[11]https://www.hawaii.edu/news/2021/04/06/ocean-deepest-point-grad-student/
[12]https://www.fws.gov/sites/default/files/documents/2024-06/appendix-1-mtmnm-mp-v.6.3.24.pdf
[13]https://deepseachallenge.com/the-science/marianas-trench-marine-national-monument/
[15]https://today.ucsd.edu/story/researchers_identify_mysterious_life_forms_in_the_extreme_deep_sea