The abandoned tunnel network under London built in secret by the Post Office, BT and the MoD
https://www.mylondon.news/news/zone-1-news/abandoned-tunnel-network-under-london-19099557
Did you know there are 4,000,000 km of secret tunnels and chambers hidden under London that, until recently, no-one knew even existed?
London is full of incredible secrets but the fact that such a huge number of tunnels and networks were kept hidden for such a long time is quite surprising.