Many years ago radium-226 and radon-222 were used as gamma-ray sources for industrial radiography : for instance, a radon-222 source was used to examine the mechanisms inside an unexploded V-1 flying bomb, while some of the early Bathyspheres could be examined using radium-226 to check for cracks. Because both radium and radon are very radiotoxic and very expensive due to their natural rarity, these natural radioisotopes have fallen out of use over the last half-century, replaced by artificially created radioisotopes. Radon therapy sits on the edge of radioactive quackery and genuine radiotherapy in part due to the lack of reliable data on the stated health benefits.