Editing Submersible (section)

=== Buoyancy control ===
During underwater operation a submersible will generally be [[Neutral buoyancy|neutrally buoyant]], but may use positive or negative buoyancy to facilitate vertical motion. Negative buoyancy may also be useful at times to settle the vessel on the bottom, and positive buoyancy is necessary to float the vessel at the surface. Fine buoyancy adjustments may be made using one or more [[variable buoyancy pressure vessel]]s as  [[trim tank]]s, and gross changes of buoyancy at or near the surface may use ambient pressure [[ballast tank]]s, which are fully flooded during underwater operations. Some submersibles use high density external ballast which may be released at depth in an emergency to make the vessel sufficiently buoyant to float back to the surface even if all power is lost, or to travel faster vertically.