One difficulty is that the researchers saw natural seeps of methane gas almost everywhere they looked. (sciencemag.org)
Only a handful were natural seeps from underground formations, and one was a vent from a coal mine, according to researchers at NASA's Jet Propulsion Laboratory and the California Institute of Technology. (canadianbusiness.com)
Experience with natural seeps in Italy suggests that any CO2 leaking from proposed underground storage is unlikely to kill (scientificamerican.com)