A huge "pumice raft" drifting west through the Pacific Ocean left no water visible, two sailors recalled.
Shannon Lenz and Tom Whitehead told the BBC they came across the 150 square kilometre - over 20,000 football fields - floating pumice in early August.
"At first they mistook it for whale poop but then it became obvious there was way more of it than any normal size pod of whales could come up with," Mr Whitehead said.
"When it got to be quite thick there was no water visible. It was just a field to the horizon on all sides," Ms Lenz added.
She said the volcanic rocks sounded like a cement mixed against their boat.
"It was just grinding. That's how we knew we were out of it because it was dark when we got out of and then it and then the noise stopped and we could hear the water again."
The couple weren't the only ones to encounter the light, bubble-rich rock.
It was first reported by Australian sailors Michael Hoult and Larissa Brill, the BBC reported.
"The rubble slick went as far as we could see in the moonlight and with our spotlight," Mr Hoult said.
Experts say the mass probably came from an underwater volcano near Tonga which erupted around August 7 according to satellite images.