The hunt for this geocache will bring you near a location that once housed a nuclear fallout shelter.
Webster dictionary defines fallout as: The often radioactive particles stirred up by or resulting from a nuclear explosion and descending through the atmosphere; also : other polluting particles (as volcanic ash) descending likewise.
Back in the Cold War days, fall out shelters were very common places found in community, government, and school and education buildings. Often private citizens built vaults and shelters in their own backyards and was a booming business during these Cold War years as nuclear war was very close to happening.
Fallout shelters were marked by a distinct yellow sign. These shelters today are mostly long gone or converted into storage areas or filled in. Every once in awhile, you may still find the sign on the exterior of a building.
You are looking for a camouflaged bison tube.