It was all little rivulets inside the place.

Camp?
Huh! It'd make a better extra reservoir for the town water-works, that place would!" "Too bad!" muttered Greg.

"I have had a notion that it would be huge fun to camp out in such a place." "I've got another idea about that," spoke up Dan.
"Fire away!" begged Reade.
"A cousin of mine who visited me last summer told me about the kind of camp he and some of his chums had.

It was a sort of manufactured cave.
The fellows dug an oblong hole in the ground.

Just like a cellar in shape, you know.