In article , Roland Perry
writes
I thought the Northern Line Edgware Branch was deepest. I know when I
used to travel via Hampstead/Golders Green my ears used to pop.

That's the deepest under the surface, but the surface is a hill! Air
pressure on the surface there will be lower as a result, by about 2.5%

I think the deepest below sea level (from memory) is the Jubilee between
Westminster and Waterloo.

Based on a 2015 FOI request, the following platforms are at least 15
metres below sea level (numbers are metres above LU datum, which is
exactly 100 metres below the OS datum at Newlyn):

74.0 Waterloo (Jubilee)
74.6 Westminster (westbound Jubilee)
76.8 London Bridge (Jubilee)
79.5 Southwark
80.2 Elephant & Castle (Bakerloo)
80.4 London Bridge (Northern)
81.9 Charing Cross (Jubilee)
81.9 Holborn (westbound Piccadilly)
82.8 Waterloo (Northern)
83.5 South Kensington (westbound Piccadilly)
83.9 Pimlico
84.0 Westminster (eastbound Jubilee)
84.4 Canary Wharf (Jubilee)
84.4 Kennington (City branch)
84.4 Bank (Northern)
84.6 Kennington (Charing X branch)
84.6 Elephant & Castle (northbound Northern)
84.7 Elephant & Castle (southbound Northern)

--
Clive D.W. Feather