On 15/07/2016 10:52, GB wrote:

The graphic didn't show how they did that bit, though.

Dig a pit either side, using an item called a "mole", burrow under the
wreck (by applying compressed air to it a weight slams back and forward
inside the mole causing it to move forward) they send this under the
wreck when it emerges in the pit at the other side the cutting wire is
attached to the air line, as the air line is pulled back through, the
cutting wire is pulled under the wreck.

OK dry land and a domestic application but it is the same idea.
https://www.youtube.com/watch?v=glpsaCSvZsI