Unsure what the structures behind the fenced enclosure are but guess they are related to the gas pipeline. A sign on the fence warns of H2S thus the cache name and info.
Hydrogen sulfide (British English: hydrogen sulphide, but this spelling is not recommended by the International Union of Pure and Applied Chemistry nor the Royal Society of Chemistry) is the chemical compound with the formula H2S. It is a colorless, flammable gas that can be identified in relatively low concentrations by a characteristic rotten egg odor.
Hydrogen sulfide often results from the bacterial breakdown of organic matter in the absence of oxygen, such as in swamps and sewers; this process is commonly known as anaerobic digestion. H
2S also occurs in volcanic gases, natural gas, and some well waters. The human body produces small amounts of H2S and uses it as a signaling molecule.
Dissolved in water, hydrogen sulfide is known as hydrosulfuric acid or sulfhydric acid, a weak acid.