Once during a reactor SCRAM drill aboard my sub in the 80's, we lost depth control and began to sink backwards into the deep. The only thing that saved us was blowing the main ballast tanks (not something you want to do while underway because it can be heard for hundreds of miles). Something similar to this is what we were told happened to the Thresher except that their hull valves through which the high pressure air flowed to the ballast tanks froze up.