LISBON, Portugal (AP) — Researchers say they may have identified a new kind of plastic pollution in the sea and they're calling it "plasticrust."

Scientists working on Madeira, a volcanic Portuguese island off northwest Africa, have found small patches of what looks like melted plastic encrusted on rocks along the shoreline. They first spotted the mostly blue and gray patches of various sizes in 2016. They are now reporting that the area the patches cover has increased substantially since then.

Tests showed the material is polyethylene, the world's most widely used plastic.

The crusts, on a specific part of the shore, are exposed at low tide.

Scientists at Portugal's Marine and Environmental Sciences Centre say they don't know yet where the plastic comes from or how it could affect marine life.