Here’s why:

[video]https://www.youtube.com/watch?v=sFODb2Ajm6Y[/video]

…and in relation to the unexploded bombs, here is a interactive map of all the recorded bombs dropped in WW2.

http://bombsight.org/#12/51.5051/-0.0900

Zoom out for fun times.

This is a fun site:

http://mappinglondon.co.uk/

I was looking for a zoomable tube/sewer/other formerly secret underground bits map but no-one seems to have done one.

So basically Londons guff!

There are maps of underground bits, but not anything AFAIK that covers the whole. One good place is Subterranea Britannica – lots of articles about stuff, you know, down there – points at floor.
Unsurprisingly the majority are not in the public realm. There is an accessible government GIS site that has quite a lot of information on it, but I don’t have the link handy.

See the fun when your building site has a Victorian cemetery under it. Every body dug up and all the soil sifted for body fragments before a pile is sunk. Oh the expense as the bodies are re-interred elsewhere. An interesting H&S exercise too (Horse hair regulations 1900 or so were consulted as this was to reduce disease from infected Crimean Hair).

Zoom out for fun times.

I knew there was a reason for the M25! It’s a demarcation line warning people not to venture inside because of things that go BOOM!