Yeah, and no one did it afterward for a long time, at which point, the depth of the Trieste’s descent right to the seafloor turned out to have been slightly less deep than measured at the time. This is a little creepy, check it out:
[snip] After passing 9,000 metres (30,000 ft), one of the outer Plexiglas window panes cracked, shaking the entire vessel. The two men spent twenty minutes on the ocean floor. The temperature in the cabin was 7 °C (45 °F) at the time. While at maximum depth, Piccard and Walsh unexpectedly regained the ability to communicate with the support ship, USS Wandank (ATA-204), using a sonar/hydrophone voice communications system. At a speed of almost 1.6 km/s (1 mi/s) – about five times the speed of sound in air – it took about seven seconds for a voice message to travel from the craft to the support ship and another seven seconds for answers to return. [/snip]
https://en.wikipedia.org/wiki/Trieste_(bathyscaphe)#The_Mariana_Trench_dives
Given the stress and material fatigue, I wonder could a sub like Trieste have returned to that depth or is this a once or twice event and then you have serious limits placed on the sub?