Does anyone know if it could surface by blowing ballast tanks (unlikely), or was it strictly maneuverable up-and-down by diving planes? If so, a stoppage of the motive power would disable diving planes and would leave it stuck wherever it was when the motive power quit.... right?
Terry, 230RN
Generally speaking submersibles (as apposed to submarines) do not use ballast tanks with variable seawater volumes to adjust buoyancy. That system is used on submarines, because they are self sufficient, and have a lot more variables that effect buoyancy, and so need a more adjustable system. Submarines also don't travel at the depths where they would need 10k psi of air to blow the tanks.
Submersibles, on the other hand, are short duration vehicles that require surface support, and can dial their weight in for the load and water conditions each dive, and need less adjustability. In general (specific details on the Titan are hard to find) submersibles will be ballasted so as to be neutral, or very slightly buoyant at the target depth of the dive. This will put them at slightly more buoyant during the decent/ascent (because they are slightly bigger at the surface and displace more water). They will use their vertical thrusters to overcome this and descend and to hold them at depth. This is done so that if they DO lose all power, they will slowly surface on their own. Additionally, the metal ballast that is used to set buoyancy can be dropped from inside the craft, resulting in significant positive buoyancy on the craft and a one way trip to the surface.
So if she's not bobbing around on the surface somewhere, odds are very high that she's either hooked on something, or there was a catastrophic structural failure that compromised the hull, and she filled with water. Assuming even a modicum of sense from her designers.