The narrow tunnel opens out on a much wider, more rounded tunnel, inclined steeply upwards for a few hundred feet, then completely blocked by a huge mass of solidified lava. Originally a volcanic vent, the tunnel runs straight and true, at least until it was blocked by cooling lava when the volcano became less explosive. The solidified lava batholith is studded with large crystals of kimberlite, quartz, and peridotite imbedded in the usual, finer-grained granite. The curving walls of the tunnel are made of darker diorite, indicating that this area was once an ocean floor. 'Contents: Loadout