It is unclear what they find in the ground, though the government is shown moving in to cover it up. It may be alien technology. It also perhaps could be government technology. Since the boys don't know, we don't know for sure. It helps us relate to the characters because we feel some of the mystery and have the same questions they do. In a video of Max Landis at Comikaze 2013, he explains what was in the hole, which would have been explained in the sequel had Fox gone with his draft.