Colossal coral in the Mariana Islands is largest of its kind

A NOAA researcher swims in front of the massive coral in the Mariana Trench Marine National Monument. Credit: NOAA Fisheries.

In a time when coral bleaching, disease, and habitat loss are increasingly common, a hidden giant defies the odds. A majestic cathedral-like structure - built by colonies of Porites rus, a species of stony coral - rises from a submerged volcanic caldera in the Maug Islands in the Mariana archipelago.

"This coral was so big, we actually couldn't easily measure it due to dive safety restrictions," said Thomas Oliver, Ph.D., a chief scientist of NOAA's National Coral Reef Monitoring Program.

A NOAA researcher swims in front of the massive coral in the Mariana Trench Marine National Monument. Credit: NOAA.

While the coral's existence was previously known to locals, NOAA scientists recently had the opportunity to take the first approximate measurements during the 2025 National Coral Reef Monitoring Program surveys.

Measurements suggest the colony covers roughly 14,500 square feet (1,347 square meters) - stretching more than 100 feet (31 meters) across the top and 200 feet (62 meters) at its base. That's wider than the length of two school buses at the top, and the length of four school buses at the bottom.

Approximate scale of the coral colony, measuring wider than the length of two school buses at the top and four school buses at the bottom. Credit: NOAA.

It is the largest Porites coral ever reported - measuring approximately 3.4 times larger than the massive Porites coral colony reported in 2020 in American Samoa. Size isn't the only impressive thing about this Porites rus, so is its age.

"It is difficult to tell the true age of this coral because it doesn't produce growth bands like other corals," said Hannah Barkley, Ph.D., a chief scientist of NOAA's National Coral Reef Monitoring Program. "We roughly estimate that Porites rus grows outward about a centimeter per year, so one could imagine that a colony of that size is pretty old."

At that rate, the coral could be more than 2,050 years old!

A unique underwater home

While this coral certainly is special, so is its home in the Maug caldera, which lies within the Mariana Trench Marine National Monument. The Maug caldera has intrigued scientists trying to protect the ocean for decades - from sea floor mapping expeditions in 2003, in-depth ocean chemistry studies in 2014, as well as the coral reef monitoring program's visits in 2017, 2022, and 2025.

The caldera is known as a "natural laboratory" because of its unique carbon dioxide vents. In one area gas bubbles up from the vents and creates acidic oceanic conditions, allowing scientists to study how organisms, like coral, may respond to these conditions in the future. Notably, the acidic conditions only impact habitats within a few meters of the vents, and do not impact the massive coral thriving just a few hundred meters away.

"It is remarkable to see both these extremes - a resilient and thriving mega coral, and a dead zone near the carbon dioxide vents - in the same area. Maug is truly such a special place," said Barkley.

The Mariana Trench Marine National Monument

The Mariana Trench Marine National Monument was established in 2009 and protects objects of scientific interest, including coral reef ecosystems, submerged volcanoes, and hydrothermal vents. The Monument is managed by the U.S. Fish and Wildlife Service, in coordination with NOAA and the government of the Commonwealth of the Northern Mariana Islands. The Monument's Advisory Council is working to assign a culturally appropriate name to the coral that will honor Indigenous Chamorro and Carolinian local heritage, while advancing stewardship of the marine ecosystems that support productive fisheries.

Map of Mariana Trench Marine National Monument. Credit: NOAA.

Coral reef ecosystems play a major role in ocean health - which we depend on for reliable weather patterns, food, coastal protection, and more. For the U.S. economy alone, coral reef ecosystems are worth more than $3.4 billion.

The National Coral Reef Monitoring Program is led by the National Ocean Service's Coral Reef Conservation Program.