Hello, SFIA audio listeners.

In this month's Nebula exclusive, big alien theory,

we're asked at 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.

Today we'll be talking about walking cities, cities on the go, because there are no clickbait

titles on this show.

So today's topic is mobile cities, and we'll be covering everything from small sediments to

big-moving metropolises, from

those floating on sea or in the sky, to those running around on giant legs like a spider

tank or big tank treads, like the behemists we see in the film Mortal Engines.

Now to some degree, we don't need to know the reasons why we might want a city to be

mobile, just how to make it work.

It's such a cool concept that we'd be interested in looking at it regardless of whether

it's practical or not.

That said, there are some compelling reasons that we might want to have a mobile city,

either cultural or pragmatic, and we'll get to those later.

We'll take a look at some of the examples in science fiction,

from the crazy to the more plausible, but we will also ask how we could make these things

...