INTERNATIONAL LUNAR EXPLORATION PHASE
- with low-cost transport, where would international astronauts go? -

Impacts from large asteroids can cause melting of the lunar rock thereby producing lava. Like on Earth, this lava can flow like a stream with the surface cooling faster than the inside stream of the lava which can then drain out. The result is a lava tube which is empty inside. Later through natural collapses or meteorite impacts, the roofs of the lava tubes can collapse creating rilles or pits or break open creating skylights.

	Depressions: Small areas of collapse can create depressions. These are not terribly interesting compared to the other lava tube phenomena. But they do indicate the presence of a lava tube beneath which, in theory, could be broken into artificially.
	Large pits: Sometimes larger lava tubes collapse their roof yet don't necessarily open up a skylight.
	Skylights: When a collapse or an impact occurs over a lava tube a hole can be broken into it. Astronauts could rappel into the lava tubes through these skylights. These missions would be particularly interesting as we watch the crew drop down and we follow them by video in real time.
	Fractures: Occasionally, lava tubes can break open with a crack.
	Arches: Rarely, a lava tube can experience two collapses next to each other leaving an arch between them. These won't be grand arches like we find on Earth due to erosion. But their rarity would make their exploration particularly interesting.