system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:When they tested nukes underground in the U.S. and elsewhere, where does all the radiation go?

What happens to all that underground fallout?
gpt
The dangerous and whidespread radiation does not come from the actual impact point of the nuke itself, the range is actually pretty small, the problem is that highly radioactive particles are spread out by the blast, and those will then, at some point (depending on what atom it is), decay and cause the radiation in the process. (gamma radiation is the only one that can travel really far, but the one that is created during the initial explosion is gone after seconds, and from then on it's the gamma radiation of those particles that are spread out across the area during the explosion). These particles are known as "the fallout", and that's how you can get rid of it: you can imagine it like a thin layer of dust settling over everything in a massive area, that you can wipe off again (of course, if it were as easy as cleaning your desk nuclear explosions would not be that much of a problem, but the general idea is the same). 

Underground the Fallout is not able to spread very far in the first place, as there is not air to travel through, but meters over meters of whatever it is that's around (most likely walls of the test facility, reeinforced with absorbing materials, and then the earth on the other side of those walls, i dont actually know what those facilities look like, or if there are any and they dont just dig a hole, put the bomb at the bottom, and then shovel all the dirt back in and detonate... if it's underground, there's somehting around it that's harder to get through than air, that's the point :P). Because the fallout cant spread out very far, it's limited to a very small area, that is a lot more realistic to clean up again, or, depending on how deep down you are, you can just leave it there, as it wont really hurt anyone, radiation is incredibly bad a getting through a lot of earth.