Gravity-powered energy storage is not new. Pumped-storage hydroelectrical plants, which pump water from one reservoir to another, have been providing utility-scale storage since the 19th century. Currently there are more than 100,000 megawatts (MW) of pumped storage hydroelectric plants operating worldwide. But with restrictions on the availability and use of water, dam construction, and other environmental concerns, it is difficult to construct new pumped-storage facilities. This is occurring at a time when the need for energy storage is skyrocketing, especially due to the increased reliance on carbon-free intermittent renewable energy supplies such as solar and wind.