Radon gas. Homeowners hear about this gas quite often, and many feel like the danger of it is exaggerated, but that is far from the case. In fact, radon is the second leading cause for lung cancer, exceeded only by cigarette smoke, so it makes sense that you should protect yourself.
Radon is invisible and odorless, so special equipment is required to detect it. Since it comes up from the ground below your home, your basement will usually have the highest levels.
To remedy the situation, it might be necessary to install fans and dig out trenches underneath your home to funnel the gas away to the outside. This can be very costly, and serves as a reminder as to why it is so important to have radon testing before buying a house. If you are fortunate, however, it might be enough to patch up holes and cracks in your basement to prevent the gas from seeping through.