Alternative building materials

[MarkP]

Seen many giant boulders here? No. The lava deposition here results in multiple harder layers alternating with layers of ash and cinders. Hawaii mountains are not good for rock climbing because of the generally "rotten" texture of the rock. Ironically the hardest chunks are made of blue rock, one of the hardest rocks in the world. This is exactly the opposite of what you would want for tunneling through. Those examples linked to are probably dug into sandstone which is very homogeneous and much easier to dig.