The cost of hydroelectric power is generally among the lowest and most stable of all electricity generation technologies when evaluated over the long lifespan of a facility. While the initial capital expenditure required to construct utility-scale dams, tunnels, and turbine stations involves massive upfront investments, the ongoing operating and maintenance costs are exceptionally low because water is a free, naturally replenishing fuel source. Once construction debts are amortized, hydroelectric plants provide exceptionally cheap electricity compared to volatile fossil-fuel plants.