Maryland has exactly one show cave, and it was found by a road crew who were not looking for it. In 1920 a quarrying operation on what is now Maryland Route 34 was drilling to place explosives when the drill bit vanished into a void. They had hit a cave. The blasting that followed produced the entrance still used today, and the cave opened to the public in 1922.
It sits between Boonsboro and Antietam National Battlefield, which makes it an easy addition to a battlefield day, and it is small enough to be honest about: a mapping survey in 1968 found around half a mile of passage, and the tour covers roughly a third of that in thirty to thirty five minutes.
Geologically it is a solutional cave formed in karst within a syncline of Tomstown Dolomite. It runs essentially horizontal throughout, and the passages are typically high and narrow. Much of the system is packed with brown and red clay, so thoroughly that opening the tour route meant digging out four to six feet of it along nine hundred feet of passage, and sediment still fills many undeveloped passages to within a foot or two of the ceiling, leaving crawlways. There is no stream in the cave. A small pool is kept topped up by drip water and nothing else.
What Crystal Grottoes is known for is density. For its size it carries an exceptional concentration of formations, mostly stalactites, flowstone and columns, generally white or buff, with an area called Fairyland tinted light red by iron oxides. The first room lies eight feet below the entrance house, an oblate chamber about thirty feet long. Tours are walk in rather than reserved, and the cave holds a steady temperature around the mid fifties Fahrenheit.
Other places on the map within a couple of hours.
