They’re the (occasional) bane of beachgoers: tar balls – small, dark pieces of oil that annoyingly stick to one’s feet while walking in the sand, and which are difficult to clean. For some others, tar balls can cause rashes or allergic reactions. But where do they come from?

Recently the US Coast Guard began searching for tar balls that began showing up on South Florida beaches from Port Everglades to Palm Beach along the coast of the Atlantic Ocean, temporarily closing some locations. But so far, the source of the black muck remains a mystery.

According to the US National Oceanic and Atmospheric Administration, tar balls 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.

“While some tar balls may be as large as pancakes, most are coin-sized,” the agency says in a fact sheet. “Tar balls are very persistent in the marine environment and can travel hundreds of miles.”

For most people, occasional brief contact with a small amount of oil isn’t harmful, officials said. But for people who are particularly sensitive to chemicals, tar balls can cause rashes or allergic reactions.

In Florida, Coast Guard crews conducted searches by air and by sea, but they were unable to find a specific source.

It’s believed that whatever was washing ashore will naturally dissipate, Coast Guard Petty Officer Nicholas Strasburg said. Though the agency is no longer investigating the source of the tar balls, it is working with local communities in case they have any further concerns, he said.

Have they shown up in the past?           

Reports are not unusual around the world, including Canada, where, in October, the Canadian Coast Guard cleaned up tar balls floating to the ocean’s surface from the site of a World War II era shipwreck off the coast of British Columbia, according to the CBC. The US Army military transport ship has been leaking oil over the years since it hit a rock and sunk in 1946, the news outlet reported.

In October 2020, tar balls turned up on parts of the Atlantic Ocean shoreline in Delaware and Maryland.

In May 2019, tar balls described as “very soft” washed up on beaches on Padre Island in Texas and were getting stuck on beachgoers, the Padre Island National Seashore reported at the time.

There were also numerous reports of tar balls along U.S. Gulf Coast beaches after the 2010 explosion and oil spill involving the Deepwater Horizon offshore drilling rig.

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