Farmington, which remains the largest city on the San Juan River and in the Four Corners region, was established in 1901 and grew significantly in the 1920s as coal, oil and natural gas were discovered in the area. Uranium was discovered in the Bluff area in the 1940s, and became a major source for the US nuclear program during World War II and afterwards for domestic nuclear plants. As a consequence of the uranium boom, over 500 mining waste disposal sites continue to contaminate the San Juan River and local groundwater. One of the most polluted sites (at the former Shiprock, New Mexico mill) exhibits considerable leakage into the river. Oil spills have also contaminated the San Juan River at times; one of the largest occurred in October 1972 when over 4.5 million gallons (1 million liters) of oil leaked into the San Juan River from a broken pipeline.