Beaches in the Australian city of Sydney have reopened for swimmers after being closed earlier this week when thousands of mysterious black tar-like balls washed ashore, prompting health concerns.

Sydney’s Bondi Beach and other beaches around the Australian city were shut on Thursday as officials investigated mysterious tar balls that have washed ashore.

Sydney beaches reopened on Saturday after thousands of tar balls had been washed ashore earlier this week. Since Tuesday, little balls made up of oil, debris, and other pollutants have littered popular eastern suburbs,