What is the maximum depth ever reached in a submarine?

semi-official, Italian designed and built Bathyscaphe X went down over Ten Grand ( l0,334 feet) this is not a world record- but was a maneuverable submarine vessel! Made in Italy, like Gina Lollobrigida. c.l954- immediate forerunner of the Trieste- also Italian build. pride runs deep- Take her down to TEN GRAND- and this was on the Batteries!

Ans2 - Please note this was a 'bathyscaphe', as opposed to a submarine. The salient technical difference is that in a bathscaphe the crew are confined to a single spherical chamber. - The visible shape around it is all machinery spaces and fuel or battery chambers that the crew cannot enter. In an actual submarine, the crew can live and work throughout the complete pressure chamber which is considerably larger than a bathyscaphe.

Now, to answer the original question, I believe it's quite possible that military submarines have dived and operated to at least 1800 feet. -But no Navy will ever acknowledge that, just as they never acknowledge where their subs are. -Secrecy is the whole point of submarine operations !