London underground is prime real estate - to find a level and avoid

* the recent new massive and extensive sewer tunnels,

* the secret basement extensions of the ultra rich, multi story archival storage, vaults, etc,

* the new underground tunnels (rail / subway for US readers),

* old roman and other buried but still 'conserved' architecture,

* crypts, graves, plague pits,

* WWII UXB's etc. etc

is a hell of a 3D needle to thread - there's > 2,000 years of urban layering in that small area.

And unmapped (and changing, inadvertently diverted) rivers