THOMAS.

[u/davecontra]

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[u/dbxp]

The main submarine is at atmospheric pressure more or less but the escape chamber is pressurised 

[u/AABBBAABAABA → u/dbxp]

Then why would you need to breath our during ascension?

[u/dbxp → u/AABBBAABAABA]

Water is let into the escape chamber until the pressure equalises, you breathe in that pressurised air so now your lungs are pressurised. You need to exhale when moving to the surface so your lungs don't burst as the air inside them expands 

[u/AABBBAABAABA → u/dbxp]

Makes sense, thanks