Portions of this text were excerpted from the Vinegaroon, a monthly newspaper in Uvalde, Texas and was originally published sometime around 1970.
As far back as the Civil War, the cavern played important roles in the nation's defense. When someone discovered that decaying bat feces (guano) becomes saltpeter, the chief ingredient of gunpowder, the steps toward a full-scale mining operation were short ones for the Frio Cave. Throughout the Civil War and World War I the cave provided tons of guano, and today the actual drying kilns for the guano still stand at the entrance. Pits were carved out of rocky bed of the nearby Frio River in which to burn wood, producing charcoal for the gunpowder production. More recently, farmers extracted guano for use as fertilizer. A narrow gauge railway which extended deep into the cave was constructed for the operation. Eventually a road which extends into the second room was constructed for 4-wheel drive trucks. Due to the expense and the creation of more efficient fertilizers, the guano mining operation has ceased, although the rusty weatherbeaten skeletons of the farmers' machinery still surround the cavern's entrance.
The most ambitious operation for utilizing the cave's assets was conceived on the eve of World War II and called the "Bat Bomb" project by some. Dr. Lytle S. (Doc) Adams and his team devised a plan for fitting millions of bats with tiny incendiary devices and dropping them on Japan. At 1,000 feet a mechanism would release the bats from their bomb-like containers, which were attached to parachutes. The bats would then scatter and roost in the eaves and cracks of the (then) primarily wooden and paper buildings, setting off hundreds of thousands of fires. President Roosevelt was so impressed with the idea that he authorized it, and it was code-named "Project X-Ray". The Frio bat cave was one of four Texas caves from which Mexican free-tailed bats were to be gathered for this project. Bat traps were built at Hondo Army Field and and hauled to the caves' entrances and Marines were assigned to guard them. Two million dollars were spent on the bat bomb project before it was abruptly abandoned. The perfection of the atomic bomb, in development at the same time as Project X-Ray, brought the bat bomb research to a sudden end. Although the project was dropped on the verge of its instigation, it still remains as perhaps the most fantastic military defense plan ever devised. Note: The book, "Bat Bomb: World War II's Other Secret Weapon" by Jack Couffer, is available at the major online booksellers. You can also check your local library. It's a fascinating book, and I heartily recommend it.
If the cave's varied history isn't enough to arouse curiosity, its dimensions and structure immediately single it out as extraordinary. Five gaping holes mark the entrance, which is situated on an ordinary looking rocky hill south of Concan. The main entrance is large enough to drive a truck through. Another hole to the left is used by the millions of bats which emerge every night in a massive column. Two nearby shafts were blasted through the ceiling for easier access to guano deposits, and other smaller entrances exist on the west side. The first room of the cave is 225 feet wide and 80 feet long with a 40 foot ceiling. Its guano-laden floor slopes sharply toward a large square opening which leads to the second room, known as "the great bat den". This room is roughly twice the size of a football field and bats cover the ceiling in a mat-like layer. Its dimensions decrease considerably after the second room; however, countless fissures, branches, and crawl spaces extend from the main tunnel (see cave map).
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