The boys will come across what could be the caved-in tunnel, but it's difficult to say. The land slopes suddenly downward near the point at which Peter and Jason estimate to be the location of the caved-in tunnel, but there aren't any other indications that there's a tunnel underground. Seems like it's been this way for a while, and the terrain has had time to settle.

The lock on the gate looks old and fairly basic, and the boys will be able to discern that a key similar to the one in that picture should fit the lock.