Radon is a colorless, odorless, radioactive gas that is formed from the decay of radium. It is the most dense gas known and has a half-life of 3.8 days. Radon is found anywhere there are radium deposits in the ground and can accumulate in buildings, posing a lung cancer risk to those inside. It was first discovered in 1900 and was responsible for many miner deaths in centuries past before its radioactive properties were understood, originally blamed on mythical dwarves. Up to 22,000 lung cancer deaths occur annually in the US due to radon exposure.