From the manual
The object of Diamond Mine is to drive your Subterranean Mining Vehicle through each mine shaft, pick up the diamonds,
process them in the special Diamond Processing Area, and
take them out of the mine shaft without being killed. You begin
the game with five vehicles.
You don't have to worry about cave-ins, black-lung disease
and ordinary dangers like that. These are special mine shafts
in which many different types of monstrosities lurk. They have
no intention of letting you escape with their precious diamonds. . .and will stop you if you let them.