This long standing mining activity has vastly modified the topography of the region. After a period of abandonment and disuse, the mines were rediscovered in 1556 and the Spanish government began operating them once again in 1724. In the 19th century, companies from the United Kingdom started large-scale mining operations. In 1873, the Rio Tinto Company was formed to operate the mines. Production declined after the peak of production in 1930, and it ended in 1986 for copper mining and in 1996 for silver and gold mining. All mining ended in 2001. Increased copper prices in the 2010s led to efforts by EMED Mining to reopen the mine, but difficulties in acquiring all necessary property rights, environmental concerns, and obtaining regulatory approval delayed reopening. The mine, which employed as many as 20,000 in the past, would employ 350 people during its startup phase. Environmental concerns are centered on long disused water reservoirs which might not be able to withstand the stress of renewed waste inputs. Controversy on its nature Due to the extreme conditions of the river, there is very little in the way of life, with the exception of small amounts of microorganisms, including algae. The presence of anaerobic bacteria in the sediments is thought to contribute somewhat to the river's famously low pH (acidity), that in turn increases the concentration of dissolved heavy metals. The waters from the Rio Tinto, high in metal sulfides, provide an ideal environment for chemolithoautotrophic microorganisms, with the sulfides acting as a food source. The product of metal sulfide metabolism through oxidization is ferric iron and secretion of acidic liquid. The continuation of this process for an extended period of time is thought by some scientists to be responsible for keeping the river's pH between 2 and 2.5 in most areas. Even in the extremely acidic water, both red and green algae have been observed to thrive in relatively high concentrations. Despite algae levels in the Rio Tinto accounting for over half of the total biomass in the river, algae is understood to have minimal effects on the characteristics of the complex ecosystem. The discovery of multiple oxide terraces mediated by microorganisms at up to 60 meters above the current water level, and as far away as 20 kilometers from the current river's path, may suggest that the unusual ecosystem is a natural phenomenon since before human mining activities started in this region.