Keyhole surgery with a Kango hammer

My mother has a small arbour in her garden constructed about 12 years ago and two of the posts have rotted. The guy who put it up finished off by setting pebbles into the area around the base. Today I began the repairs and had to remove one post completely (the other can be repaired with a concrete stud) which meant cutting through the pebbles.

The result of this work has uncovered a lost cave as can be seen in the extra ( it's actually a closeup of the hole I dug but, I thought it looks like a cave!)