Mammoth Cave in Kentucky is the longest known cave system in the world, but its "depth" is relatively shallow compared to its staggering length of over 426 miles. The vertical relief of the cave—the distance between its highest and lowest points—is approximately 398 feet (121 meters). The cave system is made up of five distinct levels of limestone passages carved over millions of years. The highest point sits at an elevation of about 810 feet above sea level, while the lowest explored point, where the Echo River flows, is at roughly 412 feet above sea level. While other caves, like the Veryovkina Cave in Georgia, descend more than 7,000 feet into the earth, Mammoth Cave's significance lies in its horizontal complexity. For visitors in 2026, most tours take place in the upper and middle levels, where the temperature remains a constant 54°F (12°C) year-round, providing a stable environment for the diverse cave-dwelling species and the thousands of tourists who explore its "bottomless" pits and grand domes.