This cave is the result of volcanism during the Pliocene or Pleistocene time (within the last 13 million years). “Tubes and tunnels in basalt occur as molten lava cools to form a solid crust on the surface even as the hot lava stream continues to run out deep in the flow. As volcanic activity producing the lava ceases, the tube is drained leaving the cave or cavern.” For the tunnel to form the lava needs to be thick and syrupy and are most often found in pahoehoe flows. Malheur cave intersects the water table and so creates an underground lake.