How Far You Actually Sink
In the movies, quicksand swallows you whole. It can't. Quicksand is about twice as dense as a human body — so, like a cork in water, you float. Step in and you sink to about the waist, then stop. Drag the sliders and watch the surface settle for yourself.
Why? Archimedes. Anything floating sinks only until the weight of fluid it has pushed aside equals its own weight — no further. The fraction of you under the surface settles at exactly
submerged fraction f = (your density) / (fluid density)
Your body, lungs and all, is barely denser than water — about 1 gram per cubic centimetre. Quicksand, a slurry of sand grains held in water, comes in around 2. So f = 1 ÷ 2 = 0.5: half of you, no more, can ever go under. That's the slider's whole story. Push the fluid down toward plain water (1.00) and the waterline climbs to your chin; nudge your body above the fluid's density and then you sink — which is exactly why you can drown in a pool but not be devoured by a sand pit.
Then why is it dangerous?
Not because you go under. Because once you stop thrashing, the disturbed grains settle and pack tight around your legs, and the fluid grips. Daniel Bonn's group measured what it takes to pull a trapped foot back out of real quicksand at a gentle 1 cm/s: about 10,000 newtons — the force you'd need to lift a small car, roughly 13 times your own body weight. You're not being pulled down. You're being held.
So the killer was never the sinking. It's time: stuck fast on a tidal flat with the water coming in, or out in the sun with no way to walk free. The escape the physicists recommend is the opposite of the movie struggle — wriggle a leg slowly to let water seep back down and re-loosen the sand, and let your own buoyancy do the lifting. It will: you were always going to float.
The check
Everything the slider shows is recomputed from first principles in research/quicksand/verify.mjs (no dependencies; run it yourself). The buoyancy law f = ρ_body / ρ_fluid is exact — Archimedes' principle. With ρ_body ≈ 1.0 and ρ_quicksand ≈ 2.0 g/cm³ it gives f = 0.50, matching the Nature experiment in which human-density beads "never became more than half submerged." Mapping that half-volume onto an upright figure (the waterline you see) uses an idealised segmented body-volume model — Dempster's standard segment volumes placed at standard stature landmarks — which puts the surface at about 61% of your height, just above the navel: "the waist." That depth is a model; the half-volume it rests on is not. The extraction force (~10⁴ N ≈ a ~1,000 kg car) is the figure reported by Khaldoun et al. for their quicksand at ~1 cm/s — it depends on the speed and the specific sand, and is quoted, not re-derived here. What's idealised: uniform body density, a clean upright pose, and a single bulk density for a material whose real behaviour is a stress-dependent rheology. What's solid: you float, and you float high.