Thousands of South Korean soldiers, police and volunteers are fighting to clean up areas of beach polluted by the country's worst-ever oil spill.
Some 10,500 tons of crude oil are estimated to have poured from a super tanker after a drifting barge punched holes in its side on Friday.
The oil has been washing ashore in the scenic Taean region on the west coast.
The government has already declared a state of disaster, but has been criticised for acting too slowly.
The BBC's John Sudworth, reporting from Mallipo Beach in the Taean region, says the smell is overpowering as the thick black oil still coming in with each tide almost completely covers the wide sandy bay.
With human chains stretching from the sea to the head of the beach, buckets of crude are being passed from grimy person to grimy person, he says.
The damaged super tanker Hebei Spirit is still visible on the horizon. It has now been sealed.
According to South Korea's ministry of maritime affairs, the Hong Kong-registered vessel was at anchor when it was hit by an industrial barge which had broken free from the tug towing it.
About 9,000 people are involved cleaning up the beaches, with more joining every day. At sea, a fleet of 140 ships has been fighting to prevent more of the spill washing ashore. Helicopters are also helping the operation.
But officials say local tourism and seafood industries have already been devastated.
MARITIME OIL SPILLS
July 1979 - Greek tanker Atlantic Express spilled 287,000 tons off Trinidad and Tobago
March 1978 - Amoco Cadiz spilled 223,000 tons off Brittany, France
November 2002 - Greek-owned Prestige spilled 77,000 tons off Galicia, Spain
March 1989 - Exxon Valdez spilled 37,000 tons in Prince William Sound, Alaska, US
December 2007 Hebei Spirit spills 10,000 tons off S Korea's west coast
More than 20 million tourists a year visit the area, drawn by its natural beauty.
The area is also home to many oyster beds and seafood farms, which supply local hotels and restaurants.
There are also wider environmental worries: Mallipo, for example, is an important transit point for migrating birds such as snipe and mallards.
As investigations continue into how the accident happened, newspapers have been criticising the government, accusing it of acting too slowly.
The Korea Herald said precious time was lost, while the JoongAng Ibo said it suspected the authorities had acted "in a loafing and idle manner".
The slick is about the third of the size of the 1989 Exxon Valdez oil spill in Alaska, considered the costliest on record.
South Korea's previously largest spill happened in 1995, when 5,000 tons of oil washed onto the southern coast.