Wikipedia: https://en.wikipedia.org/wiki/DSV_Alvin
Sounds like it was (shocker) really well designed. It even sank once when a cable snapped on support boat — crew escaped, and it was recovered and retrofit. And if things go sideways, the cabin/titanium sphere could detach, floating freely up to the surface.
After hearing about OceanGate, deep sea subs sounded terrifying — but reading about this is somehow very comforting.
Alvin is an incredible little vessel. I was fascinated by it as a kid. My grandfather had a book about the exploration of the Titanic, which featured Alvin (and the Jason Jr ROV) very prominently.