Because equipment has to be disassembled,

Photographer: Bob and Vicki
From: Down in the Salt Mine...
Panasonic » Dmctz5

Because equipment has to be disassembled,

taken down piece by piece, and then reassembled (which can take months, depending on the size), once it's no longer operational, it's discarded in one of the empty shafts. Everything down there is covered with rust.