If You Fall into Quicksand, Do You Really Sink Over Your Head?
― You Float and Stop Around the Waist. Sinking Is Not the Danger
In the movies, a person with a trapped foot slides slowly under until only a hand is left, and then that vanishes too. Physics says this almost never happens. Sand mixed with water is about twice as heavy as the human body, so you float and stop around the waist. Even so, people do get stuck in the mud of tidal flats and river mouths. The real danger is not sinking but being unable to get out.
You are walking on a tidal flat at low tide when one foot suddenly sinks. When you try to pull it out, it will not budge. It feels as if someone is gripping your ankle.
You panic and brace with the other foot, and that one sinks too. The ground around you looks exactly like the sand you were walking on a moment ago.
Why did the sand suddenly turn into ground you cannot walk on? And how deep will you sink?
Two things we know
Things float in a liquid that is heavier than they are. Sand mixed with water is about twice as heavy as water. So a human body floats and stops when it is about half submerged.
Sand with water seeping up from below is just loosely piled grains. Step on it and it collapses into a liquid. Then the grains settle and pack tight, gripping your foot.
Put these two together and it becomes clear where the movie scene goes wrong. Let's look at each in turn.
Why do you stop around the waist?
When you get into a bath, your body feels lighter. The water you push aside pushes back up with a force equal to its weight. This upward push is called buoyancy.
The human body weighs about the same as water. If your lungs are full of air, you are slightly lighter than water. That is why you can just barely float in a pool if you relax.
Sand mixed with water, on the other hand, is much heavier. The sand grains themselves are about 2.6 times as heavy as water, and water fills the gaps between them. Together, they come to about twice the weight of water.
So a person enters a liquid twice as heavy as their own body. Pushing aside half the volume is enough, so you balance when about half of you is in. For a standing person, the legs and hips make up roughly half the volume. The result is that you stop somewhere between the waist and just below the chest (Figure 1, right).
Then why can't you get out?
The danger starts here. Real quicksand forms in sand with water seeping up from below. The grains are just loosely piled, with lots of gaps.
Step on it and the way the grains are stacked collapses in an instant. Without support, the sand loosens like a liquid and your foot goes in deep. But then the grains settle downward and pack tightly. The water is pushed up and out, and only tightly packed sand is left around your foot.
This packed sand will not move easily. Worse, when you pull your foot up, a gap opens under the sole. Sand cannot flow in to fill it, so the pressure in the gap drops. The atmosphere then presses down on you from above. The harder you yank, the stronger this suction becomes. As the calculation later shows, over the area of a sole it comes to nearly 2000 newtons, which is the weight of about 200 kilograms.
It looks bottomless only because the water is cloudy and you cannot see the bottom. In fact there is a layer of sand or mud, and the depth is limited. The same state can form not only in swamps but also in tidal flats, river mouths, sandy beaches with springs, and volcanic mud.
Cornstarch mixed with water goes hard when pushed fast and lets you sink when pushed slowly. Quicksand is the reverse: it loosens when stirred. Both are "powder and water," but the behaviour flips depending on grain size and how much gap there is.
So what should you do?
- Do not yank your foot out in a panicThe harder you pull, the stronger the suction. First stop moving and steady your breathing.
- Lean back and lay your back and hands out on a wide surfaceThe wider the surface, the less you sink. It spreads out the force that was concentrated on your feet.
- Wiggle your ankle side to side to let water in, and work it out little by littleWhen water flows around it, the sand loosens. Keep each movement small and take your time.
Even if you do not sink over your head, if you are stuck while the tide rises, the water level goes up and you can drown. On a tidal flat, check the tide changes before you go in. If you see someone who cannot move, do not go in to help yourself. Call 119 (Japan's emergency number). Do not go near the mud, and do not jump into the water. If they are within reach, throw them something that floats (a cooler box, a plastic bottle, a board) and wait for rescue. Never assume you can free yourself.
Summary
Sand mixed with water is about twice as heavy as the human body. That is why you float and stop around the waist, and sinking over your head almost never happens. Instead, tightly packed sand and suction refuse to let go of your foot. What decides whether you get out is not how deep you sink but whether you can stay calm and take your time.
You don't sink, but you don't come out.
So don't rush. Let the water flow in.
A sudden change in the ground from the relationship between grains and water also happens under strong shaking. Read Why does the ground turn into "water" in liquefaction? and Why does dry sand crumble but wet sand hold together? to see what decides whether sand "sets" or "loosens."
- Take two identical cups. Fill one with water only. Fill the other about four-fifths full of sand, then pour in water. Hold them in your hands, and the sand cup is clearly heavier.
- Slowly push a finger into the sand cup, then slowly pull it out. Next, pull it out quickly from the same depth. The quick pull should feel heavier.
- Try the same with cornstarch mixed with the same amount of water, changing the speed. Unlike quicksand, it gets harder the faster you move.
Do not try this in mud outdoors. A kitchen cup is enough to check it.
For those who want to know more ― terms, formulas and links to textbooksEach part is labelled by level, from junior high science to university courses
- Junior highCovered in junior high school science
- High schoolCovered in high school "Basic Physics"
- High school+Advanced high school material, or textbook column content
- UniversityUniversity specialist courses not taught in high school (geotechnical engineering, rheology)
- ResearchTopics researchers are still investigating, not yet taught as settled fact even at university
Junior highTerms: this phenomenon has names
- Buoyancy: the upward push on an object in a liquid or gas, equal to the weight of what it pushes aside. This force sets how deep you sink.
- Pore water pressure: the pressure of the water in the gaps between sand grains. When it is high, the grains press on each other less.
- Dilatancy: the property of a grain-and-water mixture whose stiffness changes with how fast it is moved. Cornstarch in water is the classic example.
Junior highHigh schoolChecking with a formula: how deep do you sink?
The question is "how deep do you sink?" So let's work out the sinking depth itself from the buoyancy formula.
| In symbols | V_sub ÷ V ＝ ρ_body ÷ ρ_sand |
| In words | Submerged volume ÷ total body volume ＝ body density ÷ quicksand density |
| Where it comes from | The balance between buoyancy (the weight of quicksand pushed aside) and body weight. It is simply Archimedes' principle. |
| V_sub | Volume of the part in the liquid (cubic metres) |
| V | Volume of the whole body (cubic metres) |
| ρ_body | Density of the body (kilograms per cubic metre) |
| ρ_sand | Density of the sand and water mixture (same unit) |
| Density of the sand grains themselves | About 2650 kilograms per cubic metre |
| Density of water | 1000 kilograms per cubic metre |
| Fraction of gaps in loosely piled sand | Roughly 0.4 (the other 0.6 is sand grains) |
| Density of the human body | About 1000 kilograms per cubic metre (slightly lighter with air in the lungs) |
| Height | 170 centimetres |
| Weight from the sand grains | 2650 × 0.6 ＝ 1590 |
| Weight from the water in the gaps | 1000 × 0.4 ＝ 400 |
| Weight of 1 cubic metre of quicksand | 1590 ＋ 400 ＝ 1990 |
| Fraction of volume submerged | 1000 ÷ 1990 ≒ 0.50 |
| Rough depth from height | 170 × 0.50 ＝ 85 |
That is 85 centimetres up from the bottom, around the waist. Legs are thinner than the torso, so the real surface may sit a little higher. Either way, no result puts you under over your head.
| Atmospheric pressure | About 101000 pascals (used as a given fact) |
| Area of a sole | About 0.02 square metres |
| Upper limit of the suction force | 101000 × 0.02 ＝ 2020 |
| Converted to weight | 2020 ÷ 9.8 ≒ 206 |
The unit is newtons, and in weight terms it is about 206 kilograms. This is an upper-limit estimate, but it shows the force is beyond what you can use to pull out one foot. That is why "pulling fast" works against you. The key is not to rush, and to let water flow in and erase the pressure difference.
High schoolHigh school+What holds the sand up is grains pressing on each other
High schoolA pile of sand stands without collapsing because the grains press against each other and create friction. When the water pressure in the gaps is high, this pressing weakens and the friction gets smaller.
High school+The force from grains actually pressing on each other is called effective stress. In Terzaghi's principle of effective stress, effective stress is the total stress minus the pore water pressure. In sand with water welling up from below, the pore water pressure is high and the effective stress approaches zero. This is the nature of quicksand, and it is explained by the same idea as liquefaction from strong shaking.
UniversityTreating it as a material whose stiffness changes over time
Quicksand changes stiffness depending on how fast force is applied. The behaviour of such materials is studied in a field called rheology, and is described by models such as the Bingham fluid, which has a yield stress. Quicksand loosens all at once under a small force (shear liquefaction), but then the grains settle and pack densely, so resistance rises sharply. A 2005 measurement reported that the force needed to pull out a foot can reach about that needed to lift a car. This is the opposite direction from dilatancy, where the material stiffens under fast deformation.
📖 Derivations and further reading: Archimedes' principle (Wikipedia) ／ Dilatancy (Wikipedia)
ResearchWhat is still not clear
- Where the tipping point is. How much force breaks the loose packing all at once? It changes with grain shape and size variation, and predicting it for a specific site is still said to be difficult.
- What happens when clay is mixed in. Tidal-flat mud contains fine clay, and the grains stick together. How much this stickiness adds to the difficulty of escaping varies with how it is measured.
- Whether dangerous spots can be identified beforehand. They look no different from the surroundings, so methods for detecting small underground water seepage from the surface are being studied.
In other words, the content of this article is also "an explanation within what is known now." All figures are rough guides assuming typical sand, and vary with the sand or mud at a given site.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science: force, pressure and density | Why you stop at the waist; buoyancy |
| High school | Basic Physics: balance of forces and buoyancy | Parts ⓪ and ② of "Checking with a formula" |
| High school+ | Physics: pressure / Basic Earth Science: ground | Section on effective stress and pore water pressure |
| University | Geotechnical engineering, rheology | Bingham fluid, shear liquefaction |
| Research | Granular physics, coastal geomorphology | The tipping point, the effect of clay |
| ― | Link to daily life | Behaviour on tidal flats and river mouths; checking the tide |
- A. Khaldoun et al., "Liquefaction of quicksand under stress," Nature vol. 437, p. 635 (2005)
- Archimedes' principle (Japanese Wikipedia; アルキメデスの原理)
- Kenji Ishihara, Soil Mechanics (土質力学), Maruzen Publishing ― chapters on effective stress, pore water pressure and liquefaction of sand
- Japan Coast Guard (海上保安庁), "Maritime Safety Report" (海上保安レポート) ― accident cases and precautions on tidal flats and shallows
*This article is a general-audience science explainer. The figures given are guides and rough estimates to help you understand the mechanism, and do not show that any particular place or situation is safe. If you enter mud or a tidal flat, or if you encounter an accident, follow the instructions of the fire department, the Coast Guard and local authorities.