The state of a submerged body whose mean density equals that of the water around it, so that its weight and the buoyant force cancel and it neither rises nor sinks. It is how a submarine holds depth, trimming its ballast against small changes in the water, and how the floats of the international Argo programme work: each parks at a drift depth of 1,000 m for about nine days, and changes depth by changing its volume rather than its mass, pumping oil from an internal reservoir into an external bladder to lower its density and rise.
Why it matters for sightings under water
Seawater’s density changes with depth, temperature and salinity. A stratified column pushes a matched body back toward the depth where it balances; in a well-mixed column nothing does, and a small mismatch becomes a steady drift. A report of an object hovering motionless under the surface, as in Submerged Egg 2002, therefore implies either a body trimmed very closely to its water or something else holding it in place. The site’s Submerged Egg Buoyancy Bench prices both for a body the size the record gives, and its sibling Trans-Medium bench prices the other half of the Unidentified Submerged Objects question, entry into the water. How deep such a body can be seen from the air is the subject of Secchi Depth.
Connections
Unidentified Submerged Objects · Submerged Egg 2002 · Secchi Depth · Five Observables