[43north]

I believe that there were two “portholes” on Trieste. They were made of very thick quartz crystal.

One of these cracked which is what made the BANG. The two scientists inside could not find any damage so they continued their mission. It was not until after they had surfaced that the damage was discovered.

Brave, brave men.

[dr_lew]

One view port. As per Wikipedia, what broke was the "outer window" that was a protective cover for it. Of course it didn't sustain pressure across its face, and I assume it broke because of compression of the entire sphere and the attached bracket that held the outer window, "pinching" it, as it were.