LONDON — The waterfront of Sidmouth, a sleepy coastal town in southwestern England, looks serene even on a winter’s afternoon. But a monster lurks beneath the calm: a mass of fat, oil and wet wipes extending for at least 210 feet.

Known as fatbergs, such greasy, compact masses are an established urban peril. The Museum of London holds a sample of an 800-foot specimen that blocked a sewer under the capital’s East End in 2017, and New York has long battled similar agglomerations.

Sidmouth, however, has only about 13,000 permanent residents, and its fatberg was discovered in a routine check under The Esplanade, a picturesque seafront road full of hotels and restaurants.

“It is the largest discovered in our service history and will take our sewer team around eight weeks to dissect this monster in exceptionally challenging work conditions,” said Andrew Roantree, director of wastewater at South West Water, the company that manages the sewers in Sidmouth and across 4,300 square miles of England, including the cities of Exeter and Plymouth.