In a 1983 scientific paper reported that over a 5-year period, 13 bears, 3 wolverines, 9 foxes, 86 mice, 19 ravens, and more than 40 small birds, a rabbit and an eagle were found dead in one valley in a preserve in Kamcahatka, Russia. There were no signs of predators, wounds or disease—the animals were mysteriously killed by an unseen source. The mystery may be explained by the fact that this particular valley is connected with vents of a volcano. The so called, Valley of Death collects invisible gases from deep underground. Unsuspecting animals who wander into the valley, breathe in high concentrations of carbon dioxide, and the suffocate.
Read more about the discovery of this place that is off limits to any human: https://www.atlasobscura.com/articles/russian-valley-of-death