[cududa]

They're going 4,000 meters/ 2.5 miles under the sea. Roughly 400 atmospheres of pressure. A quick google shows the longest commercial tether is 1,100 meters and is meant for undersea drones for power/ data.

Aside from the massive tether and engine that would need to pull it, I'd imagine it would rip the submersible to shreds at those depths

[whats_a_quasar]

The Jason ROV has a six mile tether:

https://en.wikipedia.org/wiki/Jason_(ROV)

And the Titanic was discovered by camera sled being towed by a ship.

https://en.wikipedia.org/wiki/Argo_(ROV)

So your search might have been a bit too quick. It certainly is possible possible to build a tethered crewed submersible, though this vehicle wasn't.

Credit to js2 from this comment: https://news.ycombinator.com/item?id=36396161

[esprehn]

The tether is not connected directly, it's connected to a TMS that then has a smaller cable connected to the submersible.

Ex.

https://www.researchgate.net/figure/Schematic-image-of-a-wor...

https://f-e-t.com/subsea/vehicles/tether-management-systems/

There's also this one which looks like it can get to the depths of the Titanic:

https://nautiluslive.org/tech/rov-argus

So it was certainly possible to do this with a tether, but it would have been much more expensive.

[interroboink]

Just as another data point, the Nereus[1] explored the Mariana Trench at nearly 11km depth, and had a comms tether (very thin optical fiber). According to the wiki, its tether was 40km in length total. So, it's not out of the question, I don't think.

No good for pulling it to the surface, of course, but reliable communication alone would be a great benefit.

[1] https://en.wikipedia.org/wiki/Nereus_(underwater_vehicle)

[_yb2s]

They didn't really need an expensive and complex "umbilical cord" type of system, but a simple rope could pull the sub up in an emergency, and keep it from getting lost or separated from the ship.

This sub has 4 Innerspace 1002 thrusters that make for a max total of 342*4=1,368lb of thrust... I think a useful point of reference in deciding how much pull force would be needed for an emergency recovery tether.

I think it's reasonable that a 4mm dyneema line with a breaking strength of 4,000lbs would be sufficient for emergency surfacing, and safely provide much more thrust that it's own motors. The whole sub weighs 23k lbs and is approximately neutrally buoyant.

Total weight for 3 miles of 4mm dyneema is only about 120lbs, and for simple emergency use, in the spirit of OceanGates now infamous "scrappy thriftiness" could be pulled with the type of simple/cheap hydraulic winches used for pulling crab pots, anchors, or fishing nets. 3 miles of this line would have cost about $15,000.