The ocean is not a bucket. It is a stack of layers, each too heavy to rise and too light to sink — and the seam between them is a boundary you can only watch something find.
Warm surface water is light; cold deep water is heavy. Between them the density climbs fast through a thin band — the pycnocline. A parcel of water floats to the depth where the water around it weighs exactly what it does, and there it stops.
Drop an instrument of a chosen density and it falls until the water matches it, overshoots, and bobs back — damped by drag, tightened by the steep gradient — until it hangs on its own isopycnal. Denser probes settle deeper; nothing rests on nothing.
The boundary is never still. Internal waves roll along the pycnocline — slow, tall, and hidden from the surface — heaving whole layers up and down. The hovering probe rides them, tracing a horizon no sailor ever sees.