Elevated indoor (basement) radon levels (average 8.8 pCi/L) define a "hot" belt that extends across Onondaga County, New York, and includes much of the city of Syracuse. The region is underlain by gently dipping Paleozoic sediments of diverse lithology. Homes located on the Marcellus (black) Shale derive radon from uranium that was incorporated syndepositionally in organic-rich muds formed in an anoxic marine environment. Radon from subjacent limestones and dolostones is attributed to decay of uranium introduced into these carbonate units by ground water that previously had passed through the Marcellus Shale. Uniform distribution of radon within the carbonates suggests that the uraniferous ground water descended steeply through these units toward a basal zone where dissolution of salt has enhanced permeability. The redistribution of uranium has occurred over a period of several tens of millions of years and has been responsible for widening the "hot" belt from ∼2 km (width of Marcellus outcrop) to ∼12 km.