The term 'troublesome gas' can be summarized by the crossword solution of RADON, a naturally occurring radioactive gas. RADON is a colorless, odorless, and tasteless gas that is produced from the natural breakdown of uranium in soil, rock, and water. It is found in many parts of the world and can enter buildings through cracks in the foundation or other openings. RADON is considered to be a health hazard as it can accumulate in enclosed spaces and increase the risk of lung cancer.