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Wikipedia: "After passing 9,000 metres (30,000 ft), one of the outer Plexiglas window panes cracked, shaking the entire vessel."

If anyone else is curious about why the cracked window didn't cause an implosion, I found the answer in an interview: it was a plexiglass window on the entrance tunnel, not the viewport that was part of the sphere.

"A Plexiglas window in the flooded entrance tunnel had cracked under the pressure. But Walsh and Piccard were safe inside their cabin, separated from the tunnel by a thick steel hatch."

https://spectrum.ieee.org/don-walsh-describes-the-trip-to-th...

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So the tunnel was flooded, but not permanently connected to the outside? Otherwise why would it crack?

It was a weird, early design. 90% of that "submarine" is a giant bladder that held gasoline, with a tube through the middle, and then a sphere at the bottom. There's a window at the top of the tube, and then a steel hatch separating the sphere from the tube. It's a weird design, that's just how they decided to do it back then.

It’s not really “weird” - the design allowed the crew to leave the submersible unassisted when it was on the surface, which would be optimal for safety.

The tunnel provided a route from the crew sphere to the top of the float. During the dive it was used as a ballast tank and filled with water. Compressed air was carried to blow the water out at the end of the dive.

The original design had an open conning tower, but it was found that waves could break over it and potentially flood the diving bell if its hatch was open - hence a door was fitted. The tower was supposed to be pressure equalised, so why the door’s plexiglass cracked is a puzzle - perhaps a thin sheet will crack when exposed to a very large static pressure?

I don't know if it's that weird. The design of the Deepsea Challenger (in 2012) looks similar to me: a tiny habitable sphere surmounted by a much larger vehicle on top.

https://en.wikipedia.org/wiki/Deepsea_Challenger

Acrylic has a compressive yield strength of 95 MPa, which is the sort of pressure you’d find at around 9000m under the ocean. Perhaps the way it was designed and mounted meant that simple crushing was enough to crack it?

https://www.matweb.com/reference/compressivestrength.aspx

https://www.engineeringtoolbox.com/hydrostatic-pressure-wate...

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