Hong Kong’s 10 major beaches were closed on Sunday after a palm oil spill caused lumps of congealed white mass to float up on to its shores. Local authorities raised red flags at the beaches, warning people to avoid going there till a mass clean-up project was completed. The spill was caused after two boats crashed in mainland China’s waters, marine authorities said.

Residents said they had never seen anything like this before, though Hong Kong has faced several oil spills in the past. The substance was described by a local resident as “a bit like playdough but not as nice”, the South China Morning Post reported. It started giving off a rancid smell a few hours after being discovered.

The beaches affected include Hung Shing Yeh Beach and Lo So Shing Beach on Lamma Island, along with Pui O Beach, Tong Fuk Beach and the Upper and Lower Cheung Sha beaches.

Experts said the palm oil was likely to absorb toxins from the sea, and urged people not to play with the lumps found at the beach. Hong Kong’s overcrowded beaches, which are immensely popular with tourists, have long faced concerns of pollution.