Severn Trent says the mass of oil, grease and “unflushable” items could take until June to clear.
A giant fatberg thought to weigh about 300 tonnes is clogging a sewer in Birmingham.
The mass of oil, grease and “unflushable” items such as nappies and wet wipes is blocking the system in Hodge Hill.
Severn Trent said the fatberg was likely to be one of the biggest blockages it had dealt with.
The firm said it is estimated to be 1m (3ft) high and 1,000m (1,094yds) long and could take until June to clear.
“It is a massive project and it is not resolved yet,” operations manager Scott Burgin said.
“This giant mass is the result of everyone occasionally washing and flushing the wrong things down the drains and not realising the impact that it is having.”
Severn Trent said it was alerted to the fatberg by sewer sensors and engineers were working around the clock to tackle it.
The mass is thought to weigh the equivalent of 250 family cars.
Severn Trent workers have dug into the ground to access the sewer pipe which is 8m deep (26ft) and are using pumps to remove the sewage and clean the sewer.
In the last year, the firm said it has been called to thousands of blockages across the West Midlands with three-quarters caused by people “misusing the sewer system”.
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