Here's something most of us weren’t taught in history class: some of the most sophisticated nervous system interventions ever designed were born in caves carved into cliff faces along the Waghora River.

The Ajanta Caves of Maharashtra, India — chiseled out of basalt rock over 600 years, from the 2nd century BCE through the 6th century CE — are usually talked about as an artistic achievement. Thirty caves. Extraordinary murals. Buddhist monks in robes, meditating.

But the monks who lived there weren't just meditating. They were practicing mindful breath work there — intentionally, rhythmically, in specific ways that their tradition had mapped for centuries. Pranayama. Breath as medicine.

We are now coming to understand how the caves themselves were an important part of the healing.

The stone, the air, the sound, the breath. These four variables combined into one powerful  healing practice. Science is only now catching up to why it worked.

Across India, you find the same recurring pattern: sacred practices housed in rock. The Ajanta Caves. The Ellora Caves — Hindu, Buddhist, and Jain temples carved side by side into the same cliff. The Elephanta Caves on an island off Mumbai. Cave shrines in the Deccan Plateau that predate written history.

This wasn't coincidence, and it wasn't just practicality (though monsoon protection was definitely an added bonus). In Hindu, Buddhist, and Jain traditions, caves occupy a specific spiritual significance. They are liminal spaces — thresholds between the ordinary world and something older, deeper, more elemental. The earth herself as sanctuary.

What the monks may have noticed — what they certainly experienced, even without the language to name it — is that everything about a cave changes what happens when you practice breath work inside one.

The air is different. The sound is different. And both of those things, it turns out, matter enormously to what happens in your body.

Let's start with the atmosphere, because this piece is often overlooked but truly significant.

Caves — particularly limestone caves and enclosed rock chambers — have measurably distinct air. Studies of cave environments consistently show elevated CO2 levels (carbon dioxide), sometimes 2 to 5 times higher than outdoor air, along with near-100% relative humidity and stable, cool temperatures year-round.

When you breathe air with slightly elevated CO2, your body responds by slowing and deepening the breath. CO2 is your body's primary respiratory driver — it's not low oxygen that makes you breathe, it's rising CO2 that triggers the inhale. In a cave environment, with CO2 gently elevated, the breath naturally becomes slower, longer, more deliberate. Familiar for those that practice yoga, more like what we aim for when we practice pranayama.

Research published in the journal of Yoga & Physical Therapy (Singh, 2017) documents what yoga practitioners have long intuited: elevated blood CO2 — hypercapnia — directly stimulates the vagus nerve, producing measurable increases in respiratory sinus arrhythmia (RSA), a key marker of parasympathetic activity. In other words, slightly elevated CO2 doesn't just slow the breath. It activates the vagus nerve in the best possible way: it makes the nerve more sensitive, more responsive, and more effective at its calming function.

The cave itself was actively helping the monks to breathe differently. It was nudging their physiology into exactly the slower, deeper pattern they were also trying to cultivate through their practice.

And then there's the humidity. High moisture content in cave air reduces respiratory friction, supports the nasal mucosa, and — critical to what we will explore next — optimizes the conditions in the nasal sinuses. The sinuses are the primary site of a molecule the monks had no name for, but whose effects they may have felt every time they sat down to practice (more on that in a bit).

Now layer on the acoustics (this is where we get to tie in last month’s regulation tools).

Stone walls don't absorb sound the way wood or earth do. They bounce and amplify it. In certain chambers with the right shape, the right dimensions sound resonates at frequencies that have a powerful healing effect that deepens with each repetition. Researchers studying ancient sacred sites from Lascaux to Newgrange have found that the acoustics in these spaces likely aren't accidental. The shape of the chambers amplifies and sustains exactly the frequencies researchers now associate with physiological calming.

The geology matters here too. Limestone — the rock underlying many of India's cave temple regions — dissolves easily in water. Underground rivers carve it over millennia, leaving behind chambers with curved walls, high ceilings, irregular surfaces: the ideal architecture for acoustic resonance. Even the basalt at Ajanta, harder and darker, traps sound in ways that open air simply cannot.

The Hatha Yoga Pradipika — one of the foundational texts of hatha yoga, written in the 15th century — includes detailed instructions for a practice called Bhramari. The humming bee breath. Plug your ears, close your eyes, exhale while making a sustained humming sound on the back of your throat. Notice the vibration in your skull.

Here is the part that gets me most excited — and where the science explains the power behind the spiritual practice.

Let's talk about nitric oxide (NO), because understanding that changes how we understand the power of the practices that happened in those caves.

Nitric oxide is a molecule produced primarily in the paranasal sinuses — those hollow spaces in the skull that vibrate when you hum. It is not a minor compound. Its discoverers won the Nobel Prize in Physiology or Medicine in 1998. Nitric oxide dilates blood vessels, enhances oxygen delivery to tissues, has antiviral and antimicrobial properties, supports immune function, and plays a direct role in nervous system regulation. Your sinuses produce it continuously, and it gets carried into your lungs with every nasal inhale.

In 2002, researchers Weitzberg and Lundberg published a finding in the American Journal of Respiratory and Critical Care Medicine that deserved a much bigger headline than what it got: humming increases nitric oxide production in the nasal passages by up to 15-fold compared to quiet nasal breathing.

Not a small improvement. Not an insignificant difference. A dramatic, physiologically meaningful surge in a molecule that your cardiovascular and nervous systems genuinely need.

Here's the mechanism: the physical vibration of the hum acts as a mechanical catalyst in the sinus cavities. The resonance doesn't just produce sound — it agitates the sinus tissue in a way that dramatically accelerates NO release. The longer and more sustained the hum, the more pronounced the effect. And the more the sound is confined — held inside the skull rather than projected outward — the more the resonance builds.

This is why the cave matters beyond acoustics. The stone walls send the sound back. The hum reverberates. The vibration deepens and sustains. The monk's sinuses are essentially marinating in their own resonance — and flooding their respiratory and cardiovascular systems with nitric oxide in the process.

But nitric oxide doesn't work alone. Remember the CO2-enriched cave air? Here's where it compounds.

Research shows that slow breathing — the kind naturally induced by elevated CO2 — enhances nitric oxide bioavailability. CO2 retention activates nitric oxide synthase (NOS) activity in the vascular tissue, improving the release and uptake of NO throughout the body. The cave air is literally optimizing the conditions for the practice the monks are already doing. The benefits are stacking.

And then there's acoustic resonance itself. Sound frequencies in the lower ranges — the deep hum of Bhramari falls squarely here — can entrain the nervous system. Infrasound and low-frequency vibration have measurable calming effects. When you're in a stone chamber that's bouncing your hum back at you, amplified and deepened, you're bathing your nervous system in that frequency from both inside (your own vibration) and outside (the reflected acoustics). The vagus nerve, already sensitized by elevated CO2, receives the signal from every direction at once.

The monks didn't have this vocabulary. But they had something even more persuasive: centuries of lived experimentation with what worked.

Here's how it all comes together:

Bhramari practiced in caves wasn't just sacred sound practice. It was a multi-pathway physiological healing tool. The humming surged nitric oxide 15-fold, activating the vagus nerve through cranial vibration. The cave air, elevated in CO2 and humidity, slowed the breath and made the vagus nerve more sensitive to every input. The stone amplified the resonance, deepening the acoustic and physiological effects. And the practice of nasal breathing throughout — pranayama's foundational instruction — ensured the sinuses were primed and the NO was making it into the lungs.

Each variable compounded the others. This wasn't four separate good ideas happening in the same space. This was an integrated, intentional system.

Ancient cave temple traditions across cultures — from the chanting chambers of Elephanta to the painted caves of Lascaux — may share this common thread: the resonant acoustic properties and distinct atmospheric conditions of stone were intentionally selected for the way they supported healing practices. They weren’t described as physics or biochemistry. But they were felt and passed down as sacred.

You don't need a carved limestone chamber in Maharashtra. Though, you know, invite me over if you happen to have one!

What you do need: a small, enclosed space, nasal breathing, and five rounds of Bhramari. The more enclosed the space — a bathroom, a closet, a car with the windows up — the more the resonance reflects back and compounds the effect. Your bones do the rest.

Here's how:

What you're likely to notice: a buzzing warmth in the skull. A softening of the breath. A sense of quieting inside yourself. Possibly a gentle drop in heart rate.

That vibration you feel in your skull? That's your sinuses releasing nitric oxide at roughly 15 times their resting rate. That's your vagus nerve getting a direct signal through both the acoustic vibration and the CO2 building gently in your bloodstream during the slow exhale.

Pro tip: hum in the bathroom. The tiled walls create the closest thing most of us have to a limestone cave. The resonance reflects. The effect compounds. The monks would approve.

Ancient Indian cave temples were accidental breathwork laboratories — places where the geology, the air chemistry, and the acoustics converged to amplify what the breath was already doing. The monks who practiced Bhramari (humming bee breath) inside these chambers were, unknowingly, flooding their nasal passages with nitric oxide at 15 times the resting rate, stimulating the vagus nerve through cranial vibration, and breathing air that was naturally nudging their physiology toward the slow, deep, parasympathetic state their practice was designed to cultivate. The cave air itself — elevated in CO2, near-saturated with humidity — was a respiratory catalyst. The stone walls were an amplifier. And the breath was the instrument.

Every Saturday, we explore one ancient sacred place or practice — and the science that validates it. Simple practices, real physiology, no woo required (though woo is welcome here too).

Subscribe to our newsletter so you never miss a Sacred Saturday. Then go try your five rounds of Bhramari — preferably in a small tiled room. Your vagus nerve, your sinuses, and your future self will all thank you.

With wonder and awe,

Frederique 🌿