Dr Scott said sailors first saw rising steam plumes and then noticed the large masses of floating pumice.

Scientists say the discovery is not connected with a raft of floating pumice found last year floating further north in the Pacific.

Sailors Shannon Lenz and Tom Whitehead have recalled their encounter with a huge sea of floating pumice in the Pacific Ocean in early August.

"Looks like floating pumice, sir," answered the second mate, "and I think I see palm-trees amongst it."

Floating pumice formed a layer on the ocean surface two and a half feet in thickness, through which vessels had difficulty in forcing their way.