To carry out offshore drilling an artificial drilling platform must be constructed. This artificial platform can take many forms, depending on the characteristics of the well to be drilled, including how far underwater the drilling target is. There are two basic types of offshore drilling rigs: those that can be moved from place to place, allowing for drilling in multiple locations, and those rigs that are permanently placed. Moveable rigs are often used for exploratory purposes because they are much cheaper to use than permanent platforms. Once large deposits of hydrocarbons have been found, a permanent platform is built to allow their extraction.