Texas’ Hamilton Pool Formed When an Underground Cave Collapsed

If you drive about 45 minutes west of Austin, Texas, you will find a swimming hole that looks like a secret underground lair. This is Hamilton Pool Preserve. It is a deep green pool surrounded by massive limestone cliffs and a 50-foot waterfall. But it did not always look like this. Thousands of years ago, this entire pool was completely hidden underground.

When The Roof Caved In

Hamilton Pool is actually a collapsed grotto. A long time ago, an underground river carved out a massive cavern in the soft limestone rock. The water just kept flowing out of sight beneath the surface. Over time, the rushing water ate away at the rock from the inside out.

Eventually, the cave’s ceiling grew too thin and heavy to support itself. The entire limestone roof suddenly collapsed, dropping tons of rock directly into the water. When the ceiling crashed down, it exposed the hidden river to the open sky. It left behind a giant, circular sinkhole filled with deep water.

A Waterfall That Never Stops

The remaining rock now hangs over the water like a massive half-dome. And because the river still flows over the edge of that broken roof, it creates a 50-foot waterfall. The water never completely stops running, even during the brutally dry Texas summers.

The water below is incredibly clear. Because it is fed by a natural underground spring, the temperature stays pretty cold year-round. This makes it a popular spot for people trying to escape the Texas heat.

A Delicate Ecosystem

Because of the heavy shade from the overhanging rock and the constant supply of water, the pool created its own little microclimate. It stays much cooler down in the canyon than up on the flat prairie. You can find lush ferns and thick moss growing straight out of the canyon walls.

It is a great place to cool off, but remember that the geology here is still active. The rocks are constantly shifting. Sometimes the park has to close the swimming area entirely because heavy rains cause loose chunks of limestone to fall from the ancient ceiling. But even if you cannot get in the water, you can still hike down into the canyon and see exactly what happens when an underground cave simply gives up.