If you've been to Texas and seen vast, empty fields dotted with heavy machinery, you probably know that a good deal of the country's fuel oil comes straight out of the ground, spouting through wells that drill and pump black gold from deep below the earth's surface. It's by no means cheap or easy to get all that crude oil to the surface, and because crude oil prices remain high, energy producers are increasingly turning other energy sources. In fact, one of the most attractive alternative fuel sources doesn't come from the ground at all. Instead you'll find it in open water -- and maybe even in your own fish tank.

Scientists use the term algae to refer to a large and varied group of plantlike organisms often found in water. Like plants, algae are photosynthetic: They convert carbon dioxide into food using the power of sunlight. However, algae do not have stems or roots.

To the rest of us, algae are best known as that slimy green stuff that grows on top of pond water and at the surface of dirty swimming pools. Algae are also the scourge of aquarium keepers far and wide, who wage a never-ending battle to minimize its spread throughout their tanks. In addition to ponds, lakes, pools and fish tanks, algae also grow in soil, snow and even clouds.

While they may be a gross annoyance to some, algae serve a number of important environmental functions. According to the Encyclopedia of Biological Invasions, algae produce more than 70 percent of the world's oxygen and consume large swaths of carbon dioxide.

Perhaps more important for a planet's population who are increasingly relying oil, algae can also be used to make a number of biofuels, including ethanol and biodiesel. Methods vary, but according to Esquire magazine, to make biodiesel from algae, an energy producer often begins by selecting the strain of algae it wants to use since some are more productive or efficient than others. Oilgae reports that energy companies have started to dabble with genetically engineering the most productive algae strains, a small amount of which is added to a water-filled tank along with fertilizing nutrients. The algae are then exposed to sunlight, which starts the photosynthesis process. The carbon dioxide is converted from the air into sugar and then the algae metabolize the sugar into oil, which takes about a week. By the end of this process, the algae will have created up to 60 percent of their weight in oil.

Next, the fuel maker must extract the oil. Pressing is a common extraction method. This accomplished by either using a machine to physically squeeze the oil out (think of olives being crushed and pressed to get their oil) or by using compressed carbon dioxide, which separates the oil through vaporization. The fuel maker then uses a catalyst -- sodium hydroxide and methanol, for example -- to remove oxygen from the oil, replacing it with hydrogen to create diesel biofuel.

Refiners can also convert algae oil into other forms of biofuel, including methane, gasoline and jet fuel. According to Smart Planet, jet fuel created from algae helped power a November 2011 United Airlines flight from Chicago to Houston, the first commercial domestic flight to use algae-based fuel. The airlines aren't the only ones going (slimy) green. Oilgae estimates that roughly 100 U.S. companies and an equal number of universities are currently exploring algae-based fuel production.

Are we ready to start filling our planes, trains and automobiles with pond scum propellant? Read on to find out.