It’s not enough that whole London has a complete underground system, it has a underground river too. It’s called the River Fleet and is London’s largest subterranean river. Long time ago Victorian engineers built a whole system of tunnels and chambers under London.
The river can fill up to the roof in about 30 minutes, so if you’re a traveler that wants to experience River Fleet, you should be aware of its danger, and get out as soon as the water starts rising.
When this network of tunnels were made, the river water was clean and clear, but throughout the years it has gotten rather pollute by waste and blood. So soon it became a place you could throw junk into, even dead bodies.
A lot of things were tried to be done to fix everything, but the stink was unbearable. Luckily its somewhat clean today, and you can visit the whole network, just be safe.