What do we do when we catch a sub?
Have lunch. Italian and meatball subs are the best.
Seriously though, it depends on the circumstances. In wartime, if you are unsure, you can classify with ordinance. If the torpedo explodes, it WAS a submarine. If it doesn't it might have been a whale.
In peace time, depending on who and what it is, we will ignore it, track it, or track it long enough to vector friendly aircraft (P-8A Poseidons) or fast attack submarines.
"Ceterum censeo Islam esse delendam."
Garde la Foi, mes amis! Nous nous sommes les sauveurs de la République! Maintenant et Toujours!
(Keep the Faith, my friends! We are the saviors of the Republic! Now and Forever!)
LonePalm, le Républicain du verre cassé (The Broken Glass Republican)