Good questions. I know cities sometimes dig deeper tunnels for drainage, but I wouldn’t think they would be deep enough to need decompression.
It’s not the depth, it is the pressure in front of the machine. As Axenolith pointed out, they usually pressurize the tunnel to keep the water to a minimum. So it is like being deep under water.
After looking at the picture (post #82), I do not envy the person who has to find out what it is. There has to be a trap door somewhere in the face. All the nooks and crannies will be PACKED with rubble. Once you dig through that (by hand), you only have to deal with the fact that you are underground, most likely under water, it is dark, cold, and pressurized. And assuming they have “backed it up”, you will have feet of rubble to get through, again by hand, before you even see “the object”. Even though I bet the pay to do this will be extraordinary, I would rather get a root canal.