Semi-submersible rigs consist of a deck containing the working areas and equipment and the living quarters, supported by a hull which consists of vertical columns connected to horizontal pontoons. Such rigs operate in a 'semi-submerged' position, with the lower hull ballasted down below the waterline. In the case of a dynamically-positioned (DP) semisub, the rig keeps station over the wellhead location by means of a computer-controlled thruster system. In the semi-submerged position, it provides a stable platform for drilling, due in part to its wave transparency characteristics at the water line. The 'generation' to which a semi sub belongs is often given - this indicates the period when the rig was built and, to a certain extent, its water depth and deck-load capabilities. The latest rigs are sixth-generation. A semi-submersible rig with DP capability can drill in conventional, deep and ultra-deep waters.