a cool and comfortable . By sinking a shaft in diameter down to a cavern chamber and installing a fan powered by a electric motor, Northcott's system could change out the air through the entire house about every four minutes. Tests made over successive years by means of culture media and sterile plates were considered to have demonstrated the "perfect bacteriologic purity" of the air, purportedly a benefit to those suffering various respiratory illnesses. This "purity" was explained by a natural filtration process with air drawn into the caverns through myriad rocky crevices, then further cleansing by air floating over the transparent springs and pools, the product finally being supplied to the inmates of the sanatorium. The "Limair" burned down in the early 1900s but was subsequently rebuilt as a brick building. The Luray Caverns Corporation, which was chartered by Northcott, purchased the caverns in February 1905 and continues to hold the property today. Commercialization Portions of the caverns are open to the public and have long been electrically lighted. The registered number of visitors in 1906 was 18,000, but as of 2018, approximately 500,000 guests visit each year. In 1974, the National Park Service and the Department of the Interior designated Luray Caverns as a National Natural Landmark. Luray Caverns is home to a commercial rope course and hedge maze. The maze contains 1,500 dark American arborvitae which create a path for visitors. Three museums are also on site and included with the general admission. The Toy Town Junction Museum offers an array of vintage miniature trains, dolls, and other collectible toys on display. The Car and Carriage Caravan Museum features an impressive collection of over 140 items relating to early transportation including a Conestoga wagon and an 1892 Mercedes-Benz. The Shenandoah Heritage Village is home to a collection of historic Shenandoah Valley buildings from rustic 19th-century life and the Luray Valley Museum contains many regional artifacts of significance including a 1536 Zürich Bible and a patented dog-powered butter churn. Geology The caverns are situated in the Shenandoah Valley, just to the west of the Blue Ridge of the Appalachian Mountains in Luray, Virginia. The valley extends northeast to southwest along the northwest side of the Blue Ridge. Cave Hill, above sea level, had long been an object of local interest on account of its pits and oval hollows or sinkholes (known as karst) through one of which the