Scientists have just made a huge discovery at the mouth of the Amazon — literally.

A 3,700 square mile reef has been found right under everyone’s noses, astonishing scientists especially because of its size, according to a University of Georgia statement.

The reason why no one found a reef there before is perhaps no one expected to: reefs aren’t known to grow in brackish waters filled with sediment, as the mouth of the Amazon is.

Scientists think the reef may have been there for a very long time, and back in 1977 a research paper had even speculated that there might be a coral reef there. And in 2012, a Brazilian scientists found evidence of the reef after dredging part of the area for an Amazon River study.

But the sheer size of the reef is what’s truly surprising, even if it probably looks much different than the tropical reefs like the Great Barrier Reef that are popular with scuba divers.

Reefs usually need light to fascilitate growth, but the suspended mud and sand in the area prevents that, which also means less oxygen and photosynthesis. And yet the reef not only exists, it thrives, with plenty of fish and flora covering the reef.

“Our expedition into the Brazil Exclusive Economic Zone was primarily focused on sampling the mouth of the Amazon,” Patricia Yager, an associate professor of marine sciences in UGA’s Franklin College of Arts and Sciences and principal investigator of the River-Ocean Continuum of the Amazon project, said in the statement. “But Dr. Moura had an article from the 1970s that mentioned catching reef fish along the continental shelf and said he wanted to try to locate these reefs.”

“The paper is not just about the reef itself, but about how the reef community changes as you travel north along the shelf break, in response to how much light it gets seasonally by the movement of the plume,” added Yager.