Ghost Beach (Goosebumps #22) R.L. Stine

Jerry can't wait to explore the dark, spooky old cave he found down by the beach.
Then the other kids tell him a story. A story about a ghost who is three hundred years old.
A ghost who comes out when the moon is full.
A ghost ... who lives deep inside the cave!
Jerry knows that it's just another silly made-up ghost story....
Isn't it?