… Method, systems, and techniques for determining the age of an underground space are provided. In some embodiments, determining the age of an underground space comprises taking soil samples … a second underground space, analyzing the soil samples from the plurality of surface locations to determine an amount of a chemical compound for each soil sample, and determining an age of the … Method, systems, and techniques for determining the age of an underground space are provided. In some embodiments, determining the age of an underground space comprises taking soil samples …