MELBOURNE, Australia — The first thing the sailors noticed was the smell of sulfur.

Then, Larissa Brill and Michael Hoult, a couple sailing in the southwestern Pacific Ocean, saw it: a floating mass of volcanic rocks, with some boulders as large as basketballs, blanketing the ocean as far as their boat’s spotlight could illuminate.

The rocks — a raft of pumice estimated to be as large as 200 football fields — transformed the ocean into an opaque, undulating crust. Scientists say the raft resulted from an underwater volcanic eruption near Tonga this month, and it is slowly floating toward Australia.

A giant raft of floating rock is not all that uncommon. Similar rafts occur every five or so years, scientists said, but such masses are rarely encountered up close by people.