It starts with the magma. 3-8 miles under your feet (the feet shoved into wool socks and heavy boots that shift left and right in wait), the magma churns and bubbles. A smoldering hot liquid base, melted minerals, solid rock and dissolved gases swirl together in a fiery orange and black stew. Then just add water—rain and snowfall slides down the mountain flooding the earth’s plumbing, seeping down several thousand feet (more than a kilometer) underground. Water weaves through the cracks and earthquake faults following the path of least resistance. And you stand here in a crowd of other rain-coated tourists shuffling, sniffling and checking your watch waiting for Old Faithful to blow. So here’s how it happens.

631,000 years ago, a supervolcano rumbling under Yellowstone National Park erupted, collapsing what is now the park’s center and forming a 30- by 45-mile basin. You drive through this basin with your mom rotating the map and squinting at the squiggly lines. The lines in her forehead are just as squiggly now and she looks up to remark on how in Alaska, they see this all the time. And how the felled trees sadden her, make her feel melancholy and nostalgia for a greener time. Meanwhile, classical music (racing piano, frenetic violins and cymbal crashes) is all you can find between white noise on the radio. This underscores the scenery of dense forest, open meadows and sagebrush while shaking it with agitation. Your mom asks you to pull over.

You bristle. “There’s nothing here, just trees.”

She insists so you follow the path of least resistance coasting to the shoulder, cutting the engine and flipping on your hazards. You mumble something about how unsafe this is. Cars race by you at 50 miles per hour (the speed limit’s 45). Your mom unfastens her seatbelt and pops out waving her hands in the air. “Can you believe this?!” She belts, her voice bouncing off the nearest pines. Her boots crunch against the honey-colored twigs, mulch and pinecones as she ambles in a wide loop. “Can you take a picture of me?” She asks, adding “If there’s a bear behind me, yell bear.”

“Bear!” you yell. 

“Don’t play,” she jumps. Fast forward. 

The hot springs are the most common hydrothermal features in the park. They foam at the mouth or sit still and blue clear as a mancala piece. The Abyss Pool is aptly named. A brilliant turquoise opening at the surface, its insides burrow downward looking like midnight the deeper it goes. The rain and snow that seeped underground is heated by a deep source of magma, and rises to the surface as superheated water. Hot springs have no constrictions so water rises, cools, and sinks freely. 

On a cold day like this in October, you can spot the steam clouds a mile away wind-swept and lumpy hovering over the hot springs. Up close, they look enticing, inviting even, like a jacuzzi. You want to put your hand in it but that’s not allowed. Not only would that be painful, you’d be trespassing another’s habitat. In these otherworldly pools live a community of microscopic thermophiles: archaea, bacteria and eukarya banding together in mats and munching on algae. Your mom strikes up a conversation with some red-nosed Australians. You steal away to snap a picture of a family of three in matching yellow raincoats. Standing in a row, they look like ducklings. But back to Old Faithful.

We’re all waiting for something amazing I guess. Any moment now, a fountain of water will shoot 140 feet into the air like from a whale’s blowhole, only it’s not a whale but another natural thing that has its own pulse just below: the Earth’s core. 

Geysers erupt with steaming hot water. Unlike hot springs, they have constrictions, narrow spaces in their plumbing usually near the surface. These constrictions prevent water from circulating easily to the surface where heat would escape. So instead, the heat builds.

Think of a teapot with a narrow twisty spout. Now think of it screaming. All of that heat wants to escape. The deepest circulating water at Yellowstone can exceed the surface boiling point of 199°F. The surrounding pressure also increases with depth; increased pressure exerted by the enormous weight of the overlying rock and shallow water prevents the deeper water from boiling as it heats up. 

Right now, you may need to tie your shoelaces. You may want to get a frothy hot chocolate with a mountain of whipped cream but you don’t want to miss the eruption—which they say will happen any minute now. Maybe this will be cathartic for everyone. Maybe everyone has someone they want to erupt toward. Maybe we’re all made of magma.

As the water rises, steam forms. Bubbling upward, steam expands as it nears the top of the water column until the bubbles are too large and too numerous to pass freely through the constrictions. If this were your teapot, the lid would tremble. It can only hold so much pressure. At a critical point, the confined bubbles actually lift the water above, causing the geyser to splash or overflow. This decreases pressure on the system and violent boiling results. Tremendous amounts of steam force water out of the vent. And the eruption begins.