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Caves of the Moon
jeno passes along this excerpt from New Scientist:
"A deep hole on the moon that could open into a vast underground tunnel has been found for the first time. The discovery strengthens evidence for subsurface, lava-carved channels that could shield future human colonists from space radiation and other hazards. ... The hole measures 65 meters across, and based on images taken at a variety of sun angles, the hole is thought to extend down at least 80 meters. It sits in the middle of a rille, suggesting the hole leads into a lava tube as wide as 370 meters across."
This is R. Daneel Olivaw's hideout
These are almost certainly "sinkholes" into lava tubes, where lava runs out the center of a partially frozen lava flow. (Apollo 15 showed pretty clearly that at least the Hadley Rill was a collapsed lava tube.) There are lava tubes you can visit on the big island of Hawaii.
The interesting thing to me about this is that the interior of these tubes, being far from the Sun and in a vacuum, might easily contain an appreciable amount of water ice, for the same reason that the lunar poles might, but with a much more convenient distribution across the Moon's surface.
Besides, wouldn't it be cool to explore these 3 billion year old caves?
Very low frequency radar could do this, such as the SHARAD radar used to map the subsurface water ice on Mars.
This will not be as easy as it might seem - SHARAD uses 15-25 MHz radar, or wavelengths from 1-3 meters. A 10 meter diameter tunnel (a fairly large lava tube) would only be a few wavelengths across, and thus would be hard to see.
Apollo 17 orbited a 60 meter wavelength radar system, but I don't think that this had either the surface coverage or the resolution to realistically see lava tubes.
With this finding, I expect some nation will find the money to orbit a suitable radar around the moon to hunt for more tubes.
The Moon has highlands and Mares. The highlands are old (saturated by craters at all scales) and mostly made of a type of granite, while the Mare are relatively younger (not saturated by craters at the km scale) and made of basalt lava. This basalt lava is mostly thought to have come from the late heavy bombardment - a period of massive collisions on the Moon about 3.9 billion years ago which is now hypothesized to be from a disruption of the asteroid belt from the orbital migration of the outer planets.
This isn't a kid's balloon. As I recall, the skin is about six inches thick, and made of kevlar.
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