That natural toxic gas would be carbon dioxide. The tree kill in the area indicates a high flux of this gas through the soil. Although trees produce oxygen in their leaves, their roots draw oxygen from the soil. If high contents of carbon dioxide displace too much of the oxygen, trees cannot exist. As to the hazardous toilet: A 58-year-old cross-country skier died in a snow well next to a restroom situated along the northern shore of Horseshoe Lake (may be the one in the picture), May 24, 1998. Measurements conducted shortly after the skier's death revealed a content of 70% carbon dioxide in the snow well. ("Possible asphyxiation from carbon dioxide of a cross-country skier in eastern California: a deadly volcanic hazard", http://dx.doi.org/10.1580/1080-6032(2000)011[0192:PAFCDO]2.3.CO;2) Concentrations above 15 to 30% lead to unconciousness within a few breaths. Lying unconcious in a CO2-filled depression leads to carbon dioxide intoxication plus asphyxiation, the outcome is usually considered to be death. The tree kill began after a seismic activity around that area in 1989.

Invite struck natural flow.