In Iron Mountain, located in Shasta County, California, mining operations ended in 1963 after more than 100 years of extracting copper, gold, pyrite, silver, and zinc. However, the site continues to produce one of the most corrosive waters ever recorded, with a pH lower than -3.6 and metal concentrations reaching 200 grams per liter. This extreme acidity is the result of chemical reactions involving sulfur-containing minerals. The U.S. Environmental Protection Agency (EPA) estimates that this phenomenon could persist for 2,500 to 3,000 years unless all the sulfur-containing ore is fully removed. The extensive mining activities included open-pit mining, underground tunnels, ore crushing, rail and cable transport, a cyanide leaching plant, cementation shops, ore roasting areas, and a smelter. The main deposit was composed of nearly 95% pyrite, a mineral that reacts with water and oxygen to produce sulfuric acid and dissolved iron. This process, known as acid mine drainage, is accelerated by naturally occurring bacteria like Thiobacillus ferrooxidans, which can speed up the chemical reactions by many times. The reaction also generates enough heat to create noticeable air currents within the mountain. A treatment plant built in 1994 removes 99.99% of the metals from the water and costs between $5 to $7 million annually. Gravity pipelines carry the acidic drainage from four main sources to the plant, which treats an average of 400 million gallons of water each year before discharging the treated water into Lower Spring Creek, a tributary of the Sacramento River. The residue, a metal-laden sludge, is transported to one of four drying basins. However, this system does not provide a permanent solution to the problem. The effects of acid drainage have left a lasting impact on the region, with numerous fish kills in the Sacramento River linked to the mine’s discharge. A former EPA case manager recalled that a shovel left in the greenish liquid flowing from one of the mine galleries had been half eaten by the next day. The site has been ranked third among the most dangerous places in California. The city of Redding draws its drinking water from the Sacramento River, downstream from Iron Mountain, meaning uncontrolled acid drainage could threaten the water supply. Thanks to ongoing improvements, the levels of copper and zinc in the river have been reduced by 95% compared to pre-cleanup levels. The sludge storage area is expected to reach its maximum capacity by 2030, prompting engineers to search for new locations to store the waste. Even the case managers acknowledge that continuous treatment is not a permanent solution, and better treatment systems will be needed, highlighting the ongoing need for technological advancements. As a result, Iron Mountain will remain a site that requires ongoing monitoring for many generations to come.