A deep-sea coral reef spanning six million hectares and stretching from Miami, Florida to Charleston, South Carolina, is the largest known to date, advancing scientists' knowledge of this little-known ecosystem.

The study area, located 161km off the southeast coast of the US, covers an area of near continuous deep-sea coral mounds spanning up to 500km in length and 110km in width; with an area of high-density mounds up to 254km long and 42km wide.

The largest area, nicknamed Million Mounds is made up of stony coral found at depths between 200-1,000m, with average water temperatures of 4°C. The research area discovered, collectively called the Blake Plateau, documents the ecological importance of deep-sea coral habitats, revealing their full extent and characteristics.

Scientists synthesised bathymetric data from 31 multibeam sonar mapping surveys and 23 submersible dives to produce a nearly complete map of the Blake Plateau seafloor.

Dr Derek Sowers, mapping operations manager for the Ocean Exploration Trust and lead author of the study said, "This over 10-year, multi-agency, effort to map and characterise the coral ecosystem on the US southeast coast is a perfect example of what we can accomplish when we collaborate and focus on exploring the approximately 50% of US marine waters that are still unmapped."

The results highlight how different regions of the Blake Plateau have large variations in the density, height, and pattern of coral mound formation. The study provides a methodology aimed at interpreting mapping data over large ocean regions for insights into seafloor habitats, advancing standardised approaches to classification, and supporting ecosystem-based management and conservation efforts.

Cold-water corals grow in the deep ocean where there is no sunlight and they survive by filter-feeding biological particles. Even though they are known to be important ecosystems, creating structures that provide shelter, food, and nursery habitat to other invertebrates and fish, deep-sea corals remain poorly understood.

This research provides a better understanding of how populations of deep-sea corals and species may be related across geographically separate locations which can offer insight into their resiliency. This is important for predicting the impacts of human activities on coral communities and the fisheries managers working on the project are using this information to protect the coral reefs from physical damage.

"For years we thought much of the Blake Plateau was sparsely inhabited, soft sediment, but after years of systematic mapping and exploration, we have revealed one of the largest deep-sea coral reef habitats found anywhere in the world," said Kasey Cantwell, operations chief for NOAA Ocean Exploration.

"This discovery highlights the importance of exploring our deepwater backyard and the power of interagency collaboration and public-private partnerships."

The study was part of a coordinated ocean exploration campaign involving NOAA Ocean Exploration, Ocean Exploration Trust, the University of New Hampshire, the Bureau of Ocean Energy Management, Temple University, and the US Geological Survey, with contributions from Fugro, the NOAA Deep Sea Coral Research & Technology Program, and the South Atlantic Fishery Management Council.