Underground rivers are natural watercourses that flow beneath the Earth's surface. They often form in limestone or other soluble rocks, where water erodes the rock over time, creating tunnels and caverns. These rivers can vary in size and depth, and they play a crucial role in the ecosystem by transporting water and nutrients. Many underground rivers are part of larger systems known as karst landscapes, which feature unique geological formations like sinkholes and stalactites. Some famous underground rivers, such as the Puerto Princesa Subterranean River in the Philippines, attract tourists for their stunning beauty and ecological significance.