Radon is a naturally occurring gas that commonly enters houses though cracks in foundations, gaps around pipes, or wall joints. It's present in all homes to some degree, but at high levels it acts as a dangerous carcinogen. Because the gas is colorless, odorless, and tasteless, you'll need a radon test kit like this one
to detect its presence. If your house is afflicted, mitigation can be as easy as adding ventilation or sealing cracks in the foundation.