Apologies to Sitchensis. I had the page open too long.
Iirc, if earth is an apple, the deepest we’ve ever gone is the skin of the apple.
From Wiki…
this seems to be the answer you’re looking for
Once again, for efficiency of removal and transport, nothing beats building a pond, putting a barge in it, and dredging. It’s got to be a lot more efficient to pump a mud slurry as far as you want to take it, rather than having individual drivers driving trucks a few miles to dump, then coming back empty. Presumably once the pipes are hooked up, the barge can be operated by a minimal crew.