The researchers studied how differences in the layers of rock and soil above underground reservoirs can affect the sinking of the ground. They found that when these layers are modeled as multiple layers instead of one, the predictions of sinking can be more accurate by up to 0.75 cm. The type of rock and soil in the layers also affects the sinking, with differences of up to 2 cm for larger reservoirs. This shows that the makeup of the layers above reservoirs can have a significant impact on how much the ground sinks, and more research is needed to understand this better.