I recently studied an article about predicting earthquakes and how correctly realizing that an increase in $\ce{Rn}$ gas is a sign for the earthquake saved a whole city. The following is derived from the wikipedia page for earthquake prediction:
There are reports of spikes in the concentrations of such gases prior to a major earthquake; this has been attributed to release due to pre-seismic stress or fracturing of the rock. One of these gases is radon, produced by radioactive decay of the trace amounts of uranium present in most rock.
Radon is useful as a potential earthquake predictor because being radioactive it is easily detected, and its short half-life (3.8 days) makes it sensitive to short-term fluctuations. A 2009 review found 125 reports of changes in radon emissions prior to 86 earthquakes since 1966.
However, wikipedia then attempted to prove that most of the assretions related to radon are false statements,
but all this leaves me with one question: Why radon? (Why isn't another chemical species used for identifying the danger of feasible earthquake?) Maybe things I studied were too technical for me to understand, so please be as simple as you can be.
(Feel free to edit tags)