Underground rivers are natural watercourses that flow beneath the Earth's surface, often through caves or porous rock formations. These rivers can vary in size and depth, and they play a crucial role in the groundwater system, helping to recharge aquifers and maintain ecosystems. Many underground rivers are formed by the erosion of limestone or other soluble rocks, a process known as karst topography. Famous examples include the Puerto Princesa Underground River in the Philippines and the Cueva de los Tayos in Ecuador. These rivers can create stunning geological formations and are often home to unique wildlife.