Digging to China?

Today was spent prepping the lounge ready for the plumbers tomorrow. I decided to explore the little hole that our friend had discovered when he took the fireplace out, pre lock down. I stopped when I realised I'd gone past the separated damp proof membrane. I was now finding builders rubble but at least it was dry.  There was also a second hole at the back of the fireplace. I thought it would be a good idea to leave filling the holes in, until the plumbers had completed the pipework. The cats were wondering what the heck was going on.