Meme-Free Zone

OK, looking through the various brochures on the crane site, the mystery machine is a "Hydraulic Slurry Wall Grab." This leaves me little the wiser, unfortunately, but it looks like it goes very deep (90 meters) and does whatever it does on a very large scale.

I presume it has something to do with the "membrane walls" that were mentioned in the publicity material sent out by Crossrail, but I just don't know enough about construction to explain it.