Fracking has been called a "revolution" by the oil and gas industry. But how long can the boom last? Oil and gas has been a safe place to invest your money for the last century or so. Sure, there were some booms and busts particularly when domestic supply started to run low in the mid-20th century. But for the most part, pulling crude oil out of the ground and selling it has been a pretty profitable business. Then the shale boom happened, and things got a lot more confusing. Touted as the path to American energy independence, the "shale boom" refers to the proliferation of new technologies in the last few years by fracturing rocks to release trapped natural gass. This breakthrough made it possible to extract gas from much deeper wells. It is also the source of the pejorative nickname "fracking." Read more from Kapitall: What are some of the virtues of dividend-free value stocks? The debate about the legitimacy of fracking is pretty heated, particularly in energy-rich hubs like Pennsylvania. What's not up for debate are the profits. As late as September 2013, some analysts were predicting as much as an 8% upside to companies involved in energy exploration. And that's on top of what was already a pretty hefty run. Bulls see a future where the majority of the world's energy would come from North American shale by 2020. On the flip-side are a number of environmental activists and more skeptical economists who see a major bubble happening. Since the best mines are usually targeted first, energy can create a spiral where greater and greater capital expenditures are made to access smaller and smaller reserves. We decided to build a list of speculative oil plays. To do that, we looked at ones that were making significant investments, which you can't exactly screen for.