Ridgetop is a city in Davidson and Robertson counties in the U.S. state of Tennessee. The population was 1,874 at the 2010 census. Ridgetop 36°23′47″N 86°46′22″W (36.396306, -86.772654), is located in Robertson County except for two small portions of the city that fall in Davidson County. According to the United States Census Bureau, the city has a total area of 2.9 square miles (7.5 km2), all land. There is a small man-made lake built to draw in tourism in the early 1900s. Underneath the town is "Ridgetop Tunnel", a railroad tunnel bored in the early 1900s. When completed in 1905, it was the longest free-standing (i.e., with no columnar supports) tunnel in the world.