Enough fat to fill nine double-decker buses is being removed from sewers under London's Leicester Square.
A team of "flushers" equipped with full breathing apparatus has been drafted in with shovels to dig out an estimated 1,000 tonnes of putrid fat.
And powerful jets are being used to break it down.
The operation, which began in the early hours of this morning, is claimed to be the largest-ever sewer clean-up of its kind.
The build-up is the result of years of "sewer abuse" - when anything other than water, human waste and toilet paper is put down drains - according to Thames Water.
Danny Brackley, the water company's sewer flusher, said: "We're used to getting our hands dirty, but nothing on this scale.
"We couldn't even access the sewer as it was blocked by a four-foot wall of solid fat."
The clean-up is expected to last a couple of weeks.
Thames Water spends £12 million a year clearing around 55,000 sewer blockages across London and the Thames Valley.