The U.S. Coast Guard has been searching for tar balls that began showing up on South Florida beaches this past weekend, but so far the source of the black muck remains a mystery.

The agency has been investigating reports of tar balls from Port Everglades to Palm Beach along the coast of the Atlantic Ocean, the Coast Guard said in a statement.

Some beaches in South Florida closed Saturday when tar balls washed ashore, but they've since reopened.

What are tar balls?

Tar balls are small, dark pieces of oil which can stick to the feet of beachgoers, according to the National Oceanic and Atmospheric Administration.

They often come from oil spills, but the source can also be “natural seeps” in which oil slowly escapes from petroleum reserves beneath the ocean floor, according to NOAA. That was a possible source of tar balls that washed onto the shore in the Huntington Beach area of California in March 2024, officials said.