By Kate Newman
BBC News Online, London
About 318m bricks were used more than 100 years ago to create a network of underground tunnels to transfer London's sewage across the city.
The sewers were designed by Sir Joseph Bazalgette
But now the combined system, which deals with water flushed from homes, along with incoming rainfall from drains, is in need of improvement after raw sewage flowed into the Thames when the sewers were unable to cope with flash floods.
BBC News Online's Kate Newman has been underground to see what lies beneath the city.
It feels like I am explorer wading through a river on a warm summer's day until a shaft of light hits the water and it becomes more than visible what is underfoot - sewage.
As it floats past my leg there is no denying that I am in a lot of human waste and my weak stomach can cope as long as I do not focus on it.
After climbing down about 10 metres of ladder, attached to a harness and dressed like one of the Teletubbies, I land, luckily feet first, in the sewer.
Once my eyes adjust to the darkness I find myself in a wide brick tunnel with water flowing above my knees.
Our guides tell us that the sewage has been drained off and diverted due to our visit - normally it would be over my head.
Engineer Rob Smith once found a hand grenade in a sewer
Rob Smith, who is a catchment engineer in charge of teams maintaining the sewers for Thames Water, said: "People build up their own ideas of what it is like in the sewer.
"There is sewage in there but it is also about 99.9% water. There are sort of peaks of levels as well because we are people of habit. But the amount of sewage doesn't actually change that often but the level of water does due to storms.
"When the leaves are falling they will be rushed down road gullies and then when there is a storm it all gets flushed down the sewer because it is a combined system. This can create a lot of problems."
It smells damp as we walk in a crocodile through the 145-year-old sewers designed and constructed entirely out of brick under Sir Joseph Bazalgette and run now by Thames Water.
Slug of solidified fat
Smattered over some of the bricks there are white patches which are bits of fat - this is one of the biggest problems Thames Water faces when maintaining its 67,000km of sewers.
The company has to deal with about 100 tonnes of fat, oil and grease which is dumped in the sewers each year and causes blockages in the network.
Workers once had to clear a 150ft solidified slug of hardened fat with pick axes from sewers underneath the Leicester Square area. It took them eight weeks.
Mr Smith told BBC News Online that most people's attitude to disposing of fat is "out of sight out of mind".
"People deal with it [fat] and get rid of it and obviously don't think about where it goes.
"Generally fat and stuff being disposed into the system is a 365-day-a-year, 24-hours a day job.
"We will get in there with jetting machines to break the fat up and introduce more flow to flush it along the system so we can deal with it on our site where we can contain it and it causes less inconvenience to people."
Thames Water has to maintain 67,000km of sewers
Mr Smith from Southend has worked for Thames Water for 13 years so in that time he has come across some unusual deposits.
"I've seen horses' bodies and different bits and pieces... we get called out to anything and that's what makes the job interesting," he told BBC News Online.
"Police can call saying that they think someone has put something down the drain or flushed something down the loo and we will be called to investigate."
"We used to get involved in searching the sewers for explosive devices especially around parliament and that sort of area or when dignitaries from abroad came over - we don't do it as much now."
Mr Smith's most alarming encounter occurred early one morning.
He said: "One time a guy found a hand grenade at 2am. He said: 'I think that's a hand grenade' and handed it to me.
"I took it out of the sewer and put it down on the bank, put a guide by it and called the police. Then we got back on with our work.
"Later the police said to me 'were you the guy that found the hand grenade?' I asked if it was live and they said 'you don't really want to know that'."
Any foreign objects that are introduced to the sewage need to be removed as the liquid is treated at one of Thames Water's plants where it is turned into drinking water or pumped back into the River Thames.
Mr Smith said: "Our job is to deal with human waste, everything else that gets down that shouldn't adds to the costs.
"We are a water company as well so what we dispose of in many cases goes round in the cycle so we have to treat everything."
Two weeks ago London's drainage system could not cope with heavy rain during the storms and 600,000 tonnes of untreated sewage was forced into the River Thames.
The sewage was pushed into the river instead of allowing it to flood homes and streets.
Thames Water has been working with the Environment Agency and other bodies to come up with a with a long-term solution to the problem.
They have submitted their ideas to the Department of Environment, Food and Rural Affairs and are waiting for a response.