A record-breaking "concreteberg" the size of a blue whale is blocking three Victorian sewers in central London.
Thames Water says the rock-hard mass under Hall Street in Islington is thought to weigh 105 tonnes and stretches across at least 100m (328ft).
The water company says it is the biggest blockage it has ever seen.
Experts claim it could cost hundreds of thousands of pounds and at least two months for teams to manually chip away at it, using a range of cutting tools including jackhammer pneumatic drills and high-pressure jets.
Thames Water says local residents have been warned of the work, which is due to start next week.
Tankers will also be on standby to pump out waste 24 hours a day to protect the environment, and ensure nearby homes and businesses are not flooded with sewage caused by the blockage.
Operations manager Alex Saunders said: "Normally blockages are caused by fat, oil and wet wipes building up in the sewer but unfortunately in this case it is rock-hard concrete.
"It is in there and set to the Victorian brickwork, so we need to chip away at it to get it removed.
"This is not the first time damage has been caused by people pouring concrete into our sewers but it's certainly the worst we've seen.
"It's very frustrating and takes a great amount of time and effort to resolve.
"We're now doing everything we can to deal with it as quickly as possible, making sure our customers don't have to suffer because of this mindless abuse of our network."
The company, which serves 15 million people across London and the Thames Valley, says it spends £18m clearing blockages from its sewers.
An investigation into how the concrete got into the sewer is underway.