Why should you never step into a "hollow" near a volcano or hot spring?
― Heavy gas pools low, just like water
Around volcanoes and hot springs, invisible gases seep out of the ground. Some of them are heavier than air. These heavy gases flow into windless hollows and valley floors, pooling at the bottom like puddles of water. From the outside it looks like a perfectly calm dip in the ground — but the air right at the bottom has quietly swapped for air you cannot safely breathe.
You're walking on an autumn volcano trail when a boiled-egg smell drifts past. A little further on, you spot a quiet, sheltered hollow out of the wind. "Let's have lunch there," you might think.
But the trail has signs reading "No Entry" and "Caution: Volcanic Gas." The scenery is calm, and there's no smoke in sight. It's hard to see what could possibly be so dangerous.
And yet in Japan, hikers and workers really have collapsed in hollows like this, again and again. The reason comes down to two things: the "weight" of the gas, and the limits of the human nose.
There are only two reasons a hollow is dangerous
Carbon dioxide is about 1.5 times heavier than air, and hydrogen sulfide about 1.2 times heavier. On windless days, these gases sink to the bottom of hollows and struggle to escape.
Carbon dioxide has no smell and no colour at all. Hydrogen sulfide does smell — but at high concentrations it numbs the nose, so the smell seems to vanish.
In other words, a hollow is a place where "invisible, odourless, and low-lying" all line up at once. Let's look at each in turn.
Gas flows "downward," just like water
If you fill identical boxes with different gases and weigh them, each gas comes out a different weight. If ordinary air weighs 1.2kg, carbon dioxide weighs about 1.8kg and hydrogen sulfide about 1.4kg. A heavier gas tries to slide underneath the surrounding air.
Gas coming out of a volcanic vent or the steam of a hot spring starts out hot, so at first it rises. But once it cools, the weight difference takes over. It then flows downhill, pooling in valleys and hollows. Take a look at Figure 1.
What matters here is that the layer can be thinner than a person's height. An adult standing up might be fine, yet collapse the instant they crouch down to tie a shoelace. Small children and dogs breathe the dangerous air before adults do.
Be especially careful on windless, clear mornings, after rain, and on snowy days. Without wind, the pooled gas never gets stirred up. When snow piles up, hollows in the snow or the inside of snow walls become small hollows of their own.
The more dangerous the concentration, the less useful your nose is
Carbon dioxide is a gas with no smell and no colour. Outdoor air normally contains only about 0.04% of it. At a few percent it can cause headaches and dizziness, and at around 10% it's thought you could lose consciousness within minutes. Try changing the concentration with the slider in Figure 2.
Hydrogen sulfide smells like boiled eggs or a hot spring. Because it's detectable even at very low levels, it's tempting to assume "if I can smell it, I'll notice in time." But at high concentrations, it's thought to numb the nose's smell-detecting mechanism, so the smell can suddenly seem to disappear.
In other words, "it stank a moment ago, but now I can't smell anything" is not a sign that things have become safe. If anything, it may be a sign you've entered a dangerous concentration.
In 1986, at Lake Nyos in Cameroon, Africa, carbon dioxide that had built up at the bottom of the lake suddenly burst out. The heavy gas flowed down the valley and blanketed nearby villages, killing an estimated 1,700 people. It remains the largest-scale demonstration that heavy gas "flows along low ground."
In 1997, three Japan Self-Defense Force members died from carbon dioxide pooled in a hollow during training on Mount Hakkoda in Aomori Prefecture. The same year, four hikers died from hydrogen sulfide on Mount Adatara in Fukushima Prefecture. In both cases, the spot had looked like an ordinary, quiet low-lying area.
So what should you actually do?
- Never go past a "No Entry" or "Caution: Volcanic Gas" signDon't rest or crouch down in hollows, valley floors, or dips in snow. Be especially careful on windless days.
- If a smell suddenly disappears, move to higher ground right awayIf you feel breathless, get a headache, or feel dizzy, retrace your steps and head to a well-ventilated, higher spot.
- Even if someone has collapsed, don't rush into the hollowWould-be rescuers can collapse too. Don't approach — call 119 from higher ground instead.
Seeing someone collapsed in a hollow, your instinct is to rush over. But the air at the bottom is just as dangerous for a rescuer. In past accidents, people who went to help have collapsed themselves, making things worse. Don't approach — call 119 and wait for the fire department. Before heading out, also check the Japan Meteorological Agency's volcanic alert level and volcanic gas information. On an active volcano, falling rocks are a risk alongside the gas. Bring a helmet, and if it comes to it, take shelter in a mountain hut and protect your head.
Summary
Some gases from volcanoes and hot springs are heavier than air. Once cooled, they flow downhill like water and pool at the bottom of windless hollows. Carbon dioxide has no smell, and hydrogen sulfide stops smelling once it gets concentrated enough. That's why the safest move is simply not to enter a hollow where "invisible, odourless, and low-lying" all come together.
The bottom of a hollow is a "puddle" of air.
A vanishing smell isn't a sign of safety — it's a signal to retreat.
For how volcanoes themselves work, see "Why do volcanoes erupt?", and for another gas you can't detect in time, see "Why is carbon monoxide the scariest kind of danger — the kind you can't notice?"
- Put a teaspoon of baking soda in a cup and pour in a little vinegar. It will bubble, producing carbon dioxide.
- Once the bubbling settles, tilt the cup gently "as if pouring water" and pour the invisible gas over a candle flame (do this with an adult, and be careful with fire).
- If the flame suddenly goes out, that's proof the invisible, heavy gas flowed downward and smothered it.
Do this in a well-ventilated space, with only a small amount. If you just hold the cup over the flame without tilting it, the gas won't reach the flame, so it likely won't go out.
Want to know more? ― terms, formulas, and textbook connectionsWe've labelled which level each part belongs to, from junior-high science to university-level courses
- Jr. HighCovered in junior-high school science
- H. SchoolCovered in high-school "Chemistry" or "Earth Science"
- H. School+Advanced high-school content, or textbook sidebar material
- UniversityNot covered in high school — university-level specialist content (fluid dynamics, volcanology, occupational health)
- ResearchNot yet settled even at university level — an active area of research
Jr. HighTerminology: this phenomenon has names
- Volcanic gas: Gas released from volcanoes or fumaroles. Mostly water vapour, but also contains carbon dioxide, sulfur dioxide, hydrogen sulfide, and more.
- Density: Weight per unit volume. Between two gases, the denser one sinks below the other.
- Olfactory fatigue: A reduced ability to detect a smell after prolonged or intense exposure to it. This is thought to happen especially readily with hydrogen sulfide.
Jr. HighH. SchoolChecking the numbers: how much heavier than air is carbon dioxide?
A gas's density depends on the kind of gas (the weight of its molecules) and on temperature and pressure. Let's calculate the densities of air, carbon dioxide, and hydrogen sulfide at 20°C and 1 atmosphere.
| In symbols | ρ = P × M ÷ ( R × T ) |
| In words | Gas density = pressure × mass per mole ÷ (gas constant × absolute temperature) |
| Where it comes from | It's the ideal gas law, P × V = n × R × T, rewritten in terms of density. At the same temperature and pressure, density is proportional to mass per mole. |
| ρ | Gas density (units: kg/m³) |
| P | Pressure (units: Pa. 1 atmosphere is about 101300 Pa) |
| M | Mass per mole (units: kg/mol) |
| R | Gas constant (about 8.31 J/(mol·K)) |
| T | Absolute temperature (units: K. 20°C is 293 K) |
| Mass per mole of air | about 0.029 kg/mol |
| Mass per mole of carbon dioxide | about 0.044 kg/mol |
| Mass per mole of hydrogen sulfide | about 0.034 kg/mol |
| Denominator (gas constant × temperature) | 8.31 × 293 ≒ 2435 |
| Air's numerator (pressure × mass) | 101300 × 0.029 ≒ 2938 |
| Density of air | 2938 ÷ 2435 ≒ 1.21 kg/m³ |
| Carbon dioxide's numerator | 101300 × 0.044 ≒ 4457 |
| Density of carbon dioxide | 4457 ÷ 2435 ≒ 1.83 kg/m³ |
| Hydrogen sulfide's numerator | 101300 × 0.034 ≒ 3444 |
| Density of hydrogen sulfide | 3444 ÷ 2435 ≒ 1.41 kg/m³ |
| How many times heavier than air is CO₂ | 1.83 ÷ 1.21 ≒ 1.51 times |
| How many times heavier than air is H₂S | 1.41 ÷ 1.21 ≒ 1.17 times |
Comparing 1m³ boxes, carbon dioxide ends up about 0.6kg heavier than air — roughly the weight of a plastic bottle. But even that small a difference is enough to keep it pooled at the bottom of a hollow when there's no wind.
H. SchoolH. School+Why does hot gas sink once it cools?
H. SchoolIn the formula above, density is inversely proportional to absolute temperature T. Gas fresh out of a vent, at nearly 100°C, can actually be lighter than air even if it's carbon dioxide, so it rises at first. Once it cools to match the surrounding temperature, the difference in molecular weight shows up directly as a density difference, and it starts to sink.
H. School+Gases naturally mix a little at a time through molecular motion (diffusion). The reason gas in a hollow is so slow to thin out is that diffusion on its own is extremely slow. Without wind to stir things up, a layer of heavy gas can reportedly linger for hours.
UniversityHow heavy gas "flows," and what it does to the body
A flow of gas heavier than its surroundings, creeping along the ground, is called a "gravity current" (or density current) in fluid dynamics. How well it mixes is governed by a balance between diffusion, described by Fick's law, and turbulent mixing from wind. Hydrogen sulfide is known to block the enzyme cells use for respiration (cytochrome c oxidase). So it isn't simply a matter of diluted oxygen — it acts as a potent poison in its own right.
📖 For the derivation of the formula, and further reading: Fick's law (Wikipedia, Japanese) ／ Hydrogen sulfide (Wikipedia, Japanese)
ResearchWhat's still not fully understood
- Which hollows become dangerous, and when. Gas output varies day to day, and the effects of wind and temperature are complex. Research continues into predicting build-up from terrain and weather.
- How to safely release gas from the bottom of a lake. At Lake Nyos, pipes now slowly pump up bottom water to vent the gas. How safe this has made things is still being assessed.
- The concentration at which smell disappears varies between people. The concentration at which the nose becomes numbed is thought to differ from person to person, so no one can say for certain "you'll notice it at this concentration."
In short, this article too reflects only "what's understood for now." The figures are rough guides — always defer to local signage and information from your municipality.
Textbook connections (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Jr. High | Science Yr.1 "Properties of gases," Yr.2 "Changes in the Earth's crust (volcanoes)" | Carbon dioxide being heavier than air, volcanic gas |
| H. School | Basic Chemistry "Amount of substance," Chemistry "Gas equation of state" | Calculating density = P × M ÷ ( R × T ) |
| H. School+ | Chemistry "Diffusion," Earth Science "Volcanic hazards" | Why gas resists thinning out, hot gas sinking as it cools |
| University | Fluid dynamics (gravity currents), toxicology | Gas creeping along the ground, toxicity of hydrogen sulfide |
| Research | Volcanology, disaster-prevention science | Predicting gas build-up, venting gas at Lake Nyos |
| ― | Everyday connections | Hiking, strolling around hot springs, behaviour in hot-spring towns on snowy days |
- Japan Meteorological Agency, explanatory material on "Volcanic gas" (basic volcano knowledge)
- Lake Nyos (Wikipedia, Japanese) — ニオス湖
- Hydrogen sulfide (Wikipedia, Japanese) — 硫化水素
- Japan Meteorological Agency, "Long-term trend in carbon dioxide concentration" (気象庁)
- Kling, G. W. et al. (1987) The 1986 Lake Nyos gas disaster in Cameroon, West Africa. Science 236, 169–175.
※This article is a general-audience science explainer. The figures given are rough estimates meant to help you understand the mechanism. Near volcanoes and hot springs, always follow local signage, instructions from the fire department or local authorities, and volcanic information from the Japan Meteorological Agency. If you feel unwell, move to higher ground immediately and call 119 if needed.