Giant mole?

I've now cleared most of the junk from inside the greenhouse.  I can now see the floor, and it is worryingly uneven.  There is a large boil-like mound in it, and when I lift the black plastic covering, the earth has large fissures in its surface.  I wonder if something subterranean and menacing has made its home down there.  Something with musty fur, sharp claws and breath like comfrey juice. 

The only way to find out is to start digging.