With new geothermal projects ramping up out West, some people are beginning to question the logic of breaking up the hard rock beneath Earth’s crust — especially when it’s already bordering on fragile ground or in some cases, even fault lines.

Geothermal power is usually created by tapping steam beds just beneath the surface, often near geysers or fumaroles. But increasingly, energy companies are looking to go deeper — a much as 2.5 miles below the surface of the Earth. According to the New York Times, seismologists say this can be risky.

When you create enhanced geothermal power, you’re purposefully breaking up rocks, or creating mini-earthquakes. The injection and extraction of water to make steam causes the already delicate rock to contract and fracture. Since this fracturing is necessary for the process (it allows the water to mill through the cracks, steep and transform into steam), geothermal sites are usually located in areas where the rock is weaker.