The following commentary comes from an independent investor or market observer as part of TheStreet's guest contributor program, which is separate from the company's news coverage.By Ryan Fuhrmann NEW YORK ( StreetAuthority) -- The Department of Energy estimates that the United States holds as much as 2.5 trillion cubic feet of natural gas, which is enough to meet domestic energy demand for at least a century. Great advances in extraction techniques, including fracturing, or "fracking" of shale rock formations by drilling sideways, have simply revolutionized the domestic energy industry and led to a tripling of reserve estimates during the past few years. And while natural gas prices have remained low due to the dramatic increase in production during the past few years, it hasn't stopped firms from spending big money to increase production. Deep-pocketed international investors are even lining up to get in on the action. In the past couple of weeks, Chinese, French and Japanese firms invested an estimated $8 billion in shale-rock energy formations.