What structures can grow to a vast size underground?

Underground, structures like caves, tunnels, and root systems can grow to vast sizes. Caves, formed by the erosion of limestone or other soluble rocks, can extend for miles and host complex ecosystems. Similarly, extensive tunnel systems created by animals, such as moles or ants, can cover large areas. Additionally, the root networks of trees can spread extensively underground, sometimes reaching lengths comparable to the height of the trees themselves.