buoyant condition, weighing less than the volume of water they would displace if fully submerged. To submerge hydrostatically, a ship must have negative buoyancy, either by increasing its own weight or decreasing its displacement of water. To control their displacement and weight, submarines have ballast tanks, which can hold varying amounts of water and air. For general submersion or surfacing, submarines use the main ballast tanks (MBTs), which are ambient pressure tanks, filled with water to submerge, or with air to surface. While submerged, MBTs generally remain flooded, which simplifies their design, and on many submarines these tanks are a section of the space between the light hull and the pressure hull. For more precise control of depth, submarines use smaller depth control tanks (DCTs)—also called hard tanks (due to their ability to withstand higher pressure), or trim tanks. These are variable buoyancy pressure vessels, a type of buoyancy control device. The amount of water in depth control tanks can be adjusted to hydrostatically change depth or to maintain a constant depth as outside conditions (mainly water density) change. Depth control tanks may be located either near the submarine's center of gravity, to minimise the effect on trim, or separated along the length of the hull so they can also be used to adjust static trim by transfer of water between them. When submerged, the water pressure on a submarine's hull can reach for steel submarines and up to for titanium submarines like , while interior pressure remains relatively unchanged. This difference results in hull compression, which decreases displacement. Water density also marginally increases with depth, as the salinity and pressure are higher. This change in density incompletely compensates for hull compression, so buoyancy decreases as depth increases. A submerged submarine is in an unstable equilibrium, having a tendency to either sink or float to the surface. Keeping a constant depth requires continual operation of either the depth control tanks or control surfaces. Submarines in a neutral buoyancy condition are not intrinsically trim-stable. To maintain desired longitudinal trim, submarines use forward and aft trim tanks. Pumps move water between the tanks, changing weight distribution and pitching the sub up or down. A similar system may be used to maintain transverse trim. Control surfaces The hydrostatic effect of variable ballast tanks is not the only way to control the submarine underwater. Hydrodynamic maneuvering is done by several control surfaces, collectively known