This Is Cornwall -
Divers have been working in difficult conditions surveying the life in Loe Pool, between Helston and Porthleven.
The aim is to discover what is growing in the freshwater lake, now recovering from decades of pollution: but the dive has not been easy.
Natural England and the National Trust are behind the project, assisted by personnel from Kennack Diving, including Des Glover.
Emerging from the lake's centre, the deepest part, he said: "It's so dark down there: darker than diving in the sea at midnight.
The lake bottom's like a moonscape, but there's probably more chance of seeing life on the moon."
Fellow team member Rebecca Morton-Clark said: "It was an extremely challenging environment to dive in, so different from the sea.
"In many places the lake 'floor' was so loose and unstable, like bottomless blancmange, and so we couldn't hang around in any one place for too long, but then in shallower water here and there we'd spot tiny specks of green plants between the stones.
"It was just like hunting for treasure."
Jan Dinsdale, who has been surveying the lake from the more secure base of a boat for ten years, said: "This dive survey is the first opportunity we've had to accurately map which areas of the lake still support plant life.