A 12-foot hammerhead shark, an undersea canyon that suddenly plunges 900 feet and an enormous orange-and-brown barrel sponge are only a few things in an unexplored Puerto Rican reef that U.S. scientists are using sonar and submersibles to explore.

"You could actually live in that thing it was so big," said Tim Battista, the expedition's chief scientist.

Perhaps more significant, however, is what they are not finding: brightly colored coral and many big fish, the absence of which signals a troubled ecosystem.

"We're not seeing those vibrant, rich coral communities that all of us love to see," Battista said.

via US Expedition Explores Mysterious Puerto Rican Reef | Fox News Latino.