The chief caverns are called Endless (near New Market), Grand (at Grottoes), Luray (near Luray)--the largest in Virginia, Massanutten (near Harrisonburg), Melrose (near Harrisonburg), Shenandoah (near New Market) and Skyline (near Front Royal). These caverns are of limestone formation and contain stalagmites (upward-projecting forms on a cavern floor) and stalactites (downward-projecting forms from a cavern ceiling) in diverse shapes and colors. The Blowing Cave in Bath County received its name from the inhalation of cold air during the winter and the expulsion of cold air during the summer. Burning Spring is located in Wise County and is so named because of the liquid flames which seethe through the surface of the earth in this area from unknown sources. Crystal Spring in Roanoke received its name from the approximately five million gallons of crystal water per day which likewise appear from some unknown source. The famous Natural Bridge of Virginia is considered one of the seven natural wonders of the new world. It is located near Lexington in Rockbridge County (the county so-named because of the existence of the bridge of rock) and is a bridge of stone ninety feet long and two hundred and fifteen feet high spanning a gorge cut by Cedar Creek. So unique is this formation that Indian lore relates that it was referred to as "the Bridge of God." In this same region, in Patrick County, may be seen crystals in the shape of crosses in certain rock strata. So rare is their structure and clarity that they are often called "Fairy Stones" or "Cross Stones." The Natural Tunnel located in Purchase Ridge near Big Stone Gap and Bristol is a tunnel approximately nine hundred feet long, one hundred and fifty feet wide and one hundred feet high, carved by flowing water through solid mountain terrain. The tunnel itself includes a reverse curve, and, at the present time, railroad tracks and Stock Creek waters run through it. Crabtree Falls in Nelson County, Central Virginia, is believed to be one of the highest waterfalls east of the Mississippi River. These falls are formed by a branch of the Tye River, the South Fork, descending two thousand feet below in cascade formation. The Great Falls of the Potomac, located on the boundary between Virginia and Maryland, is one of the highest waterfalls east of the Rockies with an elevation of ninety feet. The Natural Chimneys located at Mt. Solon are seven large towers of stone carved by erosion out of a mountain. These rock strata are so straight and so symmetrical that they resemble a series of chimneys suggesting their name. Two of the chimneys have tunnels carved through the bases, and cedar trees appear to grow out of the rock.