Fat, black insects that burrow?

My old neighbours left a box in our shared garden when they moved out in December, but I only just noticed it, since the weather's just picked up enough for me to venture out there for a bit of sun. It was left on the gr, and when I picked it up (via a garden fork, since it was completely soggy and rotting and the gr beneath it is completely dead), something large, fat and black vanished down a hole in the earth. I've never seen anything like it before, and it kind of caught me by surprise. Any idea what it could have been? Looked about 2 inches long, blackish in colour and kind of shaped like a woodlouse, but much rounder about the body.