The mystery didn't last long: Microsoft is the company behind the proposed $700 million data center near Des Moines, Iowa.

"Project Mountain" is an expansion of Microsoft's existing data center in West Des Moines, pushing the company's investment in the region to the $1 billion range, says Debi Durham, director of the state agency in charge of economic development. The Iowa Economic Development Authority board approved $20 million in tax credits for the project.

The company already employs around 50 people in Iowa and plans to add at least 24 more for the new data center.

Microsoft had originally purchased a plot of land big enough for the expansion, according to the Des Moines Register, which first reported the news.

The Des Moines area has long been a finance and insurance hub and serves as the headquarters for companies such as Wells Fargo, ING Group, Principal Financial Group and AvivaUSA. Now it's becoming a significant hub for data centers. Facebook announced a $1.5 billion data center in Altoona, Iowa last April, and Google already has a data center about two hours away in Council Bluffs, Iowa. Last year the Register reported that 3-4 companies were evaluating the area for potential data centers.

Like other popular sites for massive new data centers, such as rural Oregon and North Carolina, Iowa offers tax incentives, inexpensive land and cheap energy. "The biggest issue for these companies is the cost of energy," says Durham. "We have natural energy resources that are nationally competitive." Iowa doesn't charge a tax on electricity, which provides a very large savings to these companies. She says these energy tax incentives were originally created for manufacturing companies, which are still the biggest employers in the state. She also points out that 20% of the state's energy comes from wind. Facebook, for example, cited availability of wind power as a major reason it chose Altoona for its data center.