Igneococcus · Yesterday 21:00
I've been to the seafloor at 2000 m in Alvin, the US research sub, and Alvin (at least the old version, there is a new one now) carries weights that it releases in order to resurface. I assume this sub has that option too.
I work in the marine survey industry and we do surveys with ROVs (tethered remotely operated vehicles) and AUVs (untethered autonomous underwater vehicles). Both have neutral buoyancy and the AUVs carry clump weights, which can be ditched, if something goes wrong.
I assume that the submersible has either clump weights, or ballast tanks, which can be filled with compressed air, as a minimum safety measure. I was on one job when and ROV tether broke, in 600m of water and it took 12 hours to float to the surface.