Recently, a group of sailors in the South Pacific encountered something extraordinary. One minute they were sailing in water. The next, in a sea of rocks.

Floating rocks, some the size of basketballs, as far as the eye could see. Imagine their amazement!

Turns out the rocks were pumice, which was extruded by an undersea volcano near the island of Tonga.

Pumice is molten rock filled with gas bubbles, ejected at high pressure like whipped cream from a can. When it hits the seawater, it hardens instantly, trapping the gas within it, making a rock that’s lighter than water.

The pumice then floats to the surface where it forms huge rafts of rocks, from pebble size to much larger.

They’ll float until wave action breaks up the rocks as they grind against each other, or they get waterlogged and sink.

These rock “rafts” form only about twice per decade but can be enormous.

In 2012, one formed off the coast of New Zealand that was 300 miles long.

The raft from Tonga was the size of Manhattan and was floating toward the Great Barrier Reef.

The surface of pumice is rough, full of craters and crevices, making it perfect for mollusks, corals and other sea creatures to attach themselves for the ride.

Scientists think that pumice rafts like these have helped life-forms cross open oceans and start new colonies in new places. Much the way humans did on rafts of our own making.