Scientists have discovered what they say is an “unbelievable” 85-mile stretch of coral reef off the eastern coast of the United States.

Researchers uncovered the reef some 160 miles from the coast of Charleston, South Carolina, this week, while examining deep-sea ridges and mounds that divers discovered earlier this summer.

The scientists long suspected the mounds were covered in coral, researcher Erik Cordes said. When they dove half a mile under the water this week to investigate, they found “mountains of it”.

“We couldn’t find a place that didn’t have corals,” Mr Cordes told the Huffington Post.

The reef was largely composed of Lophelia – a stony type of coral that is among the most dominant in the deep sea, according to the National Oceanic and Atmospheric Administration (NOAA).

Lophelia reefs have already been discovered off the coast of Florida and North Carolina, as deep as 600 meters underwater. But researchers were not expecting to find them here, so deep under the surface and so far from the shore.

“This is a huge feature,” Mr Cordes said. “It’s incredible that it stayed hidden off the US East Coast for so long.”

Predatory corals team up to attack and eat stinging jellyfish

The discovery was part of project Deep Search, a 15-day expedition to collect information about deep-water habitats in the US Mid- and South Atlantic. It was funded and staffed by NOAA, the Bureau of Ocean Energy Management (BOEM), and the US Geological Survey.

The research vessel departed from Woods Hole, Massachusetts, last week, and scientists have been conducting daily dives using daily dives using Alvin, a human occupied submersible, ever since then. They plan to dock on 2 September.

The beauty of coral reefs Show all 6 1 /6 The beauty of coral reefs The beauty of coral reefs A third of the world's coral reefs may face extinction STERLING ZUMBRUNN/CONSERVATION INTERNATIONAL The beauty of coral reefs A third of the world's coral reefs may face extinction Hassin Ammar/AFP/Getty The beauty of coral reefs Fish swim among coral reefs Hassan Ammar/AFP/Getty The beauty of coral reefs A fish nestles among coral reefs Hassan Ammar/AFP/Getty The beauty of coral reefs A school of fish swim over a coral reef Donald Miralle/Getty The beauty of coral reefs A giant clam among coral reefs Hassan Ammar/AFP/Getty

The voyage comes as President Donald Trump proposes the expansion offshore drilling in the Arctic, Atlantic and Pacific oceans. The plan, put forward by Mr Trump and Interior Secretary Ryan Zinke in January, would open up approximately 90 per cent of the waters off the US coastline to oil exploration.

At the time, Mr Zinke told reporters that the administration had identified the largest number of sales ever proposed for the country’s offshore leasing program. He added that the plan would “strike the right balance to protect our coasts and people, while still powering America and achieving American energy dominance”.

In a statement announcing project Deep Search last fall, BOEM Acting Director Walter Cruickshank said his organisation had been planning the project since before Mr Trump took office. But he indicated that it could still be used to inform coming policies.