Adams Tunnel West Portal, Colorado

A 13-mile long ten-foot wide water tunnel runs under Rocky Mountain National Park, from one end to the other, crossing 3,700 feet under the Continental Divide. It is the fulcrum of a network of trans-basin, trans-divide reservoirs and pipelines known as the Colorado-Big Thompson project, built by the U.S. Bureau of Reclamation, and completed in 1947, at a cost of $160 million. The system involves a number of reservoirs, pipelines, and pumping stations on the western slope to collect water that would otherwise flow into the Colorado River, and away to places like California. These include the Willow Creek Reservoir and the Windy Gap Reservoir, which deliver captured water to Lake Granby, the largest of the reservoirs in the system, via pipelines and canals. At Grand Lake, the collected Colorado River water enters the Adams Tunnel through an underwater tube, and runs as a straight line for 13.1 miles under Rocky Mountain National Park, to the east portal, which is 109 feet lower in elevation from the west portal, allowing the water to flow by gravity. It gets from one end to the other in around two hours.