TAIPEI (Taiwan News) — Massive amounts of pumice that formed during Japanese submarine volcano Fukutoku-Okanoba’s eruption in August have drifted near Taiwan’s Green Island, prompting the Fisheries Research Institute to warn nearby vessels to steer clear.

According to the institute, the bed of pumice appeared around the Ryukyu Islands in early October and affected local marine traffic; now it is approaching eastern Taiwan due to the Black Current and easterly winds. As the rock floats on water, it poses a threat to vessels’ cooling systems, which may become clogged with pumice as they draw seawater to cool engines.

CNA cited the institute as saying pumice floating on the ocean’s surface is hard to spot and that vessels should alert each other about the location of the rocks. Eventually, they will wash up on shore, break down, or sink.

A Green Island resident surnamed Tien (田) told CNA that pumice had appeared along the island’s northeastern coast and waters. Locals are familiar with the rock, as Green Island was formed through volcanic eruptions, and the islanders use it to wash pots and pans and for gardening.