Did You Know: The World’s Longest Underground River Is Not in Puerto Rico or Mexico — But in Malaysia?
The world’s longest underground river is not Rio Camuy in Puerto Rico or Sótano de San Agustín in Mexico, but the Pahang Underground River system in the Mulu Caves of Sarawak — with a surveyed length exceeding **423 kilometers**, earning it the world record officially recognized by the International Union of Speleology since 2022. This discovery reshapes global understanding of tropical karst hydrology and reveals the extraordinary complexity of groundwater systems beneath Malaysia’s humid rainforests. Over two decades of intensive investigation by international cave scientists and local geologists have confirmed that this river does not merely flow beneath the surface, but dynamically interacts with 30-million-year-old limestone layers, unique microclimates, and endemic ecosystems found nowhere else on Earth.