Under a building on Toronto Island, a 30cm pipe extends underground jutting out into Lake Ontario. The pipe stretches nearly four km to a series of unworldly fabric balloons packed full of compressed air. An old marine salvage technology, the balloons, or airbags, have found a new function anchored under 70 metres of water. Storing surplus electrical energy in cheap, environmentally neutral way is the Holy Grail of engineers in the power industry. From next-generation batteries to fly wheels and thermal storage, many new technologies have emerged, but none has been chosen to supplant the traditional method known as “pumped hydro.” Mechanics of pumped hydro station are simple. Water is pumped uphill into a reservoir and held there, storing its potential energy. When more electricity is required, the water is released and funnelled downhill to power a turbine…Instead of pumping water uphill, the company pumps compressed air underwater. Hydrostor can store energy that wind turbines or solar panels produce on sunny or windy days to offset lack of energy produced on cloudy, windless days. The technology can offset the risk of blackouts on hot summer afternoons when the power grid is being stretched to its limit by air conditioners…To charge its underwater air battery, Hydrostor turns on an air compressor… sends the air underwater and stores the excess heat from the compression process for use later…To discharge, Hydrostor opens a valve for system run in reverse. “When valve is opened, weight of water around air cavity forces air to come up to lower pressure at surface and begins turns a low-pressure turbine, reproducing power.”…While many companies are working with compressed air energy storage, Hydrostor is the only company to have a functioning underwater facility.