In message , at 14:24:18 on Fri, 15 Nov
2019, Bryan Morris remarked:

I find the Picc unbearably deep, so by the time I've reached Barons
Court (westbound) I'm standing by the door gasping for fresh air. No
other tunnel affects me like that, not even the Chunnel.

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.

Later: Hampstead Station is approx 200m above sea level at the surface,
140m above sea level at the platform. Westminster and Waterloo Jubilee
Line platforms are between 25-26m below sea level.
--
Roland Perry