In London it is quite common for wealthy individuals to wish to expand their home, but that can be a problem, as even despite being wealthy, land in London is inordinately expensive, therefore in order to gain more space for their already pre-existing homes, cheaper, they will have underground floors added.
In order to achieve this contractors will use a JCB Excavator to dig the hole under the house, but once done the digger has no way of getting out of the hole it has dug, so rather than lift it out by crane they have the digger dig one more hole, and then bury it in concrete down there in the newly dug basement as it is cheaper to bury it down there, rather than have to hire a crane, pay to block of the road to get the crane in, further damage the house and pay the workers for extra time, in the end the customers tend to be the kind that can afford it, so it's cheaper to just bury it.
The same sort of thing happened with the last British tunnel boring machine (TBM) used to dig the channel tunnel, most, including all of the French TBMs, were dismantled, but the last British one instead was diverted off to the side to bore another tunnel, away from the main tunnels, once it had dug clear of the main tunnels it was powered down and bricked off, leaving it there, forever entombed, having never made it to France.
I'm willing to bet you feel bad for the excavators and boring machines, probably thinking "Aw, poor digger, left down there in the dark after working so hard." Well to that I say:
It's a digger, who cares.
Further Info: http://www.telegraph.co.uk/finance/property/news/10879282/Londons-trail-of-buried-diggers.html