Fluids like water, gas, or magma move through cracks in rocks deep underground. This flow of fluids controls many important Earth processes, including earthquakes, volcanic eruptions, and the movement of heat or valuable minerals. However, predicting how and where these fluids move is difficult because it depends on how rocks are squeezed and stretched by stresses in the Earth's crust.

In this study, we used a special machine that can apply pressure to a rock from three different directions at once. This allows us to better mimic the stress conditions found in nature. We also measured how easily fluids could flow through the rock as the pressure changed. The rock we tested was a type of basalt from Mount Etna in Italy.

We found that fluid flow first becomes more difficult as cracks close under pressure. But as the rock is squeezed harder, new cracks begin to form, and fluids suddenly start flowing much more easily, but only in a specific direction. This shows that stress alone can re-route underground fluid flow. These results can help us better understand and predict fluid behaviour in areas where earthquakes, volcanic activity, geothermal energy, or carbon storage are important.