Nine beaches in Sydney, including well-known Manly beach, were closed to bathers on Tuesday after small white and grey balls of debris washed up on the shores at the height of the summer holiday season.

Sydney’s popular Manly Beach and other beaches around the Australian city were closed on Tuesday as authorities investigated marble-sized balls of white and gray debris that had washed on shore, in the latest such mysterious incident in recent months.

Authorities in Australia say they have decided to shut nine beaches while they investigate the unusual grey and white balls.

Mysterious balls appear on Sydney beachesNorthern Beaches Council

Sydney’s iconic beaches are famous worldwide for their golden sand and pristine waters. They attract millions of tourists every year, but this time, they have drawn the attention of researchers and local authorities for an unusual reason.