A fatberg is a congealed lump of fat, sanitary disposables, wet wipes, and similar items found in public sewer systems. If not broken up, a fatberg can form a solid blockage the prevents the free flowing of sewers resulting in sewer flooding. It's said a fatberg is often created when cooking oil is poured down drains while hot and runny. Once in the cold water of the sewer system, the fat congeals and combines with wet wipes and other solid wastes. Although fatbergs are health hazards and problematic clogs in city sewer systems, they have also been identified as an alternative source of biogas fuel.

In September 2014, Thames Water, the company that keeps sewers flowing freely under London, released a set of disgusting pictures of a large fatberg that took a week to remove from a 262-foot stretch of Shepherd's Bush Road in West London. The water authority claimed it was the size of a Boeing 747, if it was buried underground.