There is a limestone cave near Naples, Italy. People call it the "Dog Death Cave".

Because this is a limestone cave. When limestone (the main component is calcium carbonate) encounters groundwater, it will decompose carbon dioxide. The bubbles that emerge from the rock cracks in the cave are carbon dioxide gas. Carbon dioxide is heavier than air and accumulates at the bottom of the cave. Dogs are shorter than humans, so they are surrounded by carbon dioxide gas. Of course, they will suffocate to death over time. People also feel difficulty breathing when they bend over and lower their heads, and matches will go out on their own, which all illustrate this problem.