Crystal Grottoes, the only commercial cave now open in Maryland, is also one of the largest in the state. Nearly one half mile of passage was mapped in 1968. The cave was discovered in 1920 as a result of quarry operations for road material. A drill bit (for placing explosives) disappeared into a hole, and the crew realized they had found a cave.
Blasting subsequently created an opening large enough to allow exploration. Because of their beauty and commercial possibilities, the caverns were spared in the quarrying and, in 1922, after the excavation of clay and silt that filled much of the cave and the installation of electrical equipment, the caverns were opened to the public. The entrance house, originally a wooden structure, was replaced in 1942 by the modern stone building.
Photo from www.facebook.com/CrystalGrottoesCaverns/