Does anyone remember this short story?
A group of scientists are studying an underground cave when they begin to see milky shapes moving in the rock. Eventually a couple of midget men appear through the rock, and are shot at by one of the scientists/explorers. They respond by firing pellets of air compressed to such an incredible degree that they literally burst one of the explorers apart.
One of the little men tries to take the body of the explorer with him back into the rock but give up. I seem to remember one the explorers/scientists theorising that the men are so dense that they pass between the rocks like water through porous sponge.
Top marks to anyone who can remember this story, as I used to visit some pretty obscure bookshops when I was younger (a mum who liked reading) and so this may not be well known story.