It uses a lot less glass than above the ground greenhouse.

Especially good protection for plants against extreme heat and cold, but in Florida this would be called a "pool" .... because our water table is too high.  

A lot more useful than a pool:

This earth-sheltered greenhouse taps into the thermal mass of the earth, so that much less energy is needed to heat up the walipini's interior than an aboveground greenhouse. Of course, there are precautions to take in waterproofing, drainage and ventilating the walipini, while aligning it properly to the sun -- which the manual covers in detail.

Best of all, according to the Benson Institute, their 20-foot by 74-foot walipnifield model out in La Paz cost around $250 to $300 only, thanks to the use of free labour provided by owners and neighbours, and the use of cheaper materials like plastic ultraviolet (UV) protective sheeting and PVC piping.

Cheap but effective, the underground greenhouse is a great way for growers to produce food year-round in colder climates. More over at the Benson Institute and the Pure Energy Systems Wiki.

Bill Gates home? 

Although he has a lot of green energy features, I'm pretty sure he has no greenhouse.