Phosphorus is the 15th element on the periodic table, and has the chemical symbol "P".
Phosphorus is essential for life and is the first new element to be "discovered" and isolated. Fun fact - phosphorus was first isolated in 1669 by Hennig Brand. He did it by boiling down urine and subjecting the reside it to a few high temperature chemical manipulations. As pure phosphorus is a pyrophoric element (it ignites spontaneously in air), ol' Hennig got quite the surprise the first time. But like any good alchemist, he didn't let a little (or big) explosion get in his way and he did it again - but this time he was ready and was able to isolate the compound in a vacuum.
Phosphorus is essential to modern agriculture and is present in almost all fertilizers. However, run-off from large farms has caused high levels of phosphorus in lakes. This is thought to be one of the main contributors to the die offs in Lake Erie in the 70s and 80s. While advances in farming and more regulation has helped Erie rebound, the P is still present in the silty bottom and took much dredging or large storms can continue to cause die offs and other problems.
Poop is high in Phosphorus - specifically bird and bat poop. Perhaps you've heard of Nauru? It's an island in the Pacific and was composed in large part of phosphate on limestone. The phosphate was "deposited" by birds over tens of thousands of years. Intensive strip mining led to extensive environmental devastation and left the island nation impoverished after the reserves were depleted in the early 1990s.
Anyway, it's a cool element and has an interesting history. You're looking for a micro cache in a cemetery. Bring your own writing utensil. No room for trades. No night caching!
References:
https://thereader.mitpress.mit.edu/dark-history-nauru/