In 1911, a 2-km3 (0.5-mi3) earthquake-triggered rock slide blocked the Murgab River, southeastern Tajikistan, forming a still-existing, 600-m-high (1,970-ft-high) natural dam—the highest dam, natural or man-made, in the world. Lake Sarez, impounded by this blockage, is 60 km (37 mi) long, with a maximum depth of 550 m (1,800 ft) and a volume of approximately 17 km3 (∼4 mi3). This lake, which has never overtopped the dam, exits the downstream face as a series of large springs that regroup as the Murgab River. Freeboard between lake surface and the lowest point on the dam crest currently is approximately 50 m (∼165 ft), and the lake is rising at an average rate of 18.5 cm/yr (7.3 in./yr). If the blockage were to fail, a worst-case scenario could endanger tens or possibly hundreds of thousands of people in the Murgab, Bartang, Panj, and Amu Darya valleys downstream. Dam failure potentially could result from: 1) seismic shaking, 2) catastrophic overtopping caused by a landslide entering the lake from the valley wall at high velocity, 3) surface erosion caused by natural overtopping by the rising lake, 4) internal erosion (piping), 5) instability caused by lake pressure against the dam, or 6) slope instability of the dam faces. Occurrence of an overtopping wave resulting from a potential landslide high on the right bank of Lake Sarez seems to be the most realistic of these slight possibilities for failure. Because of the high cost of installing physical remediation to the dam in this rugged mountain area (no roads lead to the site), the main protective measures now being undertaken are hydrological monitoring at the dam and installation of a flood early warning system downstream.