This was an illustrated children's science-fiction book I read in the early 1970s, but had a very 1960s feel (thinking back now). There were a couple of astronauts on the moon. Somehow they got into a system of strange, round tunnels. The tunnels turned out to be made by giant worms.
I don't remember anything else about the plot, but I associate the book with some astronaut figure toys I had at the same time - I'm fairly sure the toy astronauts were a tie-in to the book. The figures were about 5-6" tall, in bright orange and white spacesuits, and made of rubber over a wire skeleton, so they were very poseable. They had plastic space helmets.