MIAMI (AFP) - Two US professors will spend 73 days in an underwater laboratory near the Florida Keys, where they'll teach courses via Internet and try to break the record of the longest-ever undersea stay.

Bruce Cantrell and Jessica Fain, of Roane State Community College in the southeastern US state of Tennessee, took the plunge Friday.

They aim to stay 8m under the surface in a Key Largo lagoon at the Jules' Undersea Lodge through Dec 15, according to a site dedicated to the project.

While there, Cantrell, 63, and Fain, 25, will teach courses for their students on ocean exploration and conservation, through videos posted on a dedicated YouTube page.

They will also post interviews with researchers and experts who will visit them underwater, including former astronaut Buzz Aldrin, the second man who walked on the moon.

The experiment "will offer the most unique educational experience in the college's history and raise awareness of issues affecting the oceans," said Roane State Community College.

The pair will also try to enter the Guinness Book of World Records for the longest-ever underwater stay.

The current record holder is Richard Presley, who spent 69 days under water, from May to July 1992, also in the Key Largo area.