You get Radon gas due to the uranium decay chain. Down in the ground we have minerals like granite which contain very small amounts of uranium. The uranium decays over time, releasing Radon.

Radon then usually gathers under foundations and in our area can make its way into basements. This is usually remediated in a simple way. They're use a venting system connected to a low point in your basement, likely your sump pump pit, seal it with plexiglass, and route a PVC pipe from there to the outside of your home with a fan that runs 24/7 that continually pulls out any particles to the outside.