[HWR_14]

Leaving aside the literal tons of weight a tether would require just to life a single person back to the surface (and the weight of the tether), it also could get pushed around a lot by the current. And, most importantly, once you are down there and want to navigate the wreck it would get in the way.

[js2]

The Jason ROV has a 6-mile tether, so it's possible:

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

Titanic was also discovered with a tethered sled, the Argo:

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

It was then explored with a combination DSV and tethered ROV:

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

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

[londons_explore]

The weight of the tether doesn't matter when it is in water. You just design it to be neutrally buoyant. Either using mini pressure vessels or tanks of light oil.

[WalterBright]

Neutral buoyancy varies by depth, because water compresses. You'd have to have a tether with the same density of water and the same compressibility.

[londons_explore]

The compressibility of water is fairly negligible - the density increase is only 1% for every 2 kilometers of depth. And obviously the cable also compresses at depth - probably by a similar percentage,

[carlosjobim]

The metal wires in the tether is heavier than water. This is balanced by twining air/gas hoses into the tether. Gas compresses by 50% for each 10 metres of depth in water, so that's a whole different scale.

[londons_explore]

You either put floats (ie. Gas chambers of fixed volume) or sealed tanks of a light liquid, for example kerosene.

For high depth like this, tanks of a light liquid are probably most suitable. I'd probably use liquid butane. With a vapour pressure of 40 psi, it is easy to contain, and is has a density of only half of water. If it leaks, there won't be an environmental cleanup cost either.