Hello, SFIA audio listeners. In this month's Nebula exclusive, big alien theory,

we're asking the reason alien civilizations might be rare is because most aliens are huge.

To hear it and every episode early and ad-free, plus hours of bonus content,

check out go.nebola.tv slash Isaac Arthur and use my code, Isaac Arthur.

The moon is a harsh mistress with an airless surface racked by two-week periods of scorching

sunlight followed by the long night, but below its surface lies caverns and tunnels we

might shelter in that are greater in scope than anything on Earth.

Yeah. greater in scope than anything on Earth. Today we'll be examining the pros and cons of settling lava tubes on the moon and other places, along with asking what life inside one would be like for those early settlers.

Lava tubes are natural underground tunnels formed by flowing lava under the surface of a

planet or moon, or a larger asteroid for that matter.

They are created when the surface of a lava flow cools and solidifies, while the lava

beneath continues to flow. When the lava flow stops and drains away, it leaves behind a hollow

tube. As best as we can tell, most lunar volcanic activity was between 3 to 4 billion years

ago, but some activity was as recent as a billion years back. Normally, where Earth is considered

normal anyway, such tours would be long gone, but there is no continuing

geological activity to shake them, nor wind or rain or flood to collapse them will fill

them with sediment, nor organic processes inside to break them, as moss or tree roots can

do to stone on Earth.

And indeed on some planets, lava tubes might be the only

place life could reside or begin, but that life might damage those tunnels and self-destruct

their ecosystem in the process. This lack of destructive forces also means they can get very

large, even compared to the ones we get on Earth where they can be wide enough to drive a truck down and miles long.

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