Fool's Gold — Zilpha Keatley Snyder

For as long as he can remember, people have  warned fourteen-year-old Rudy and other kids in the  neighborhood not to play in Pyramid Hill's abandoned  gold mines. Even Rudy's best friend, Barney, a  true daredevil, won't go near those cavernous death  traps. That is, until Tyler Lewis moves in from  L.A. Ty's determined to go in, and he's got a map.