Stepping inside NRG Energy's underground cooling plant in the heart of downtown Phoenix, you feel a little like Alice falling down the rabbit hole. One minute you're standing outside an anonymous metal door next to the Phoenix Convention Center, and the next you're three stories underground, walking across a metal bridge suspended over a massive holding tank filled with bubbling, neon-green ice water. Named the Downtown Cooling System, this plant, along with two others sprinkled throughout the city core, chills and then pumps near-freezing water to more than 35 buildings across downtown via an underground system of pipes.

The water, which is dyed green in case of a leak, is then used to air-condition everything from Chase Field and US Airways Center to Symphony Hall and the Fourth Avenue County Jail.

Even cooler: The system chills CityScape's outdoor ice rink every December, and also powers the Valley's only air-conditioned Valley Metro light-rail station, at the corner of Second Street and Jefferson.