In some parts of Earth's interior, seismic waves travel at different speeds depending on the direction in which they are moving through the layers of rock in Earth's interior. This property is known as seismic anisotropy, and it can offer important information about how the silicate rock of the mantle—particularly at the mantle's lowermost depths—deforms. In contrast, areas through which seismic waves travel at the same speed regardless of direction are considered isotropic.