Related by context. Frequent words. (Click for all words.) 54 severe droughts 52 prolonged droughts 51 limestone caves 51 glacial melt 50 Omo River 50 aqueducts 50 aridity 50 Melting glaciers 49 frequent droughts 49 ancient 49 Egyptian pharaohs 49 Tibetan Plateau 49 Inca Empire 48 groundwater extraction 48 salination 48 limestone cave 48 retreating glaciers 48 droughts 48 torrential rainstorms 48 Aswan Dam 48 aqueduct 48 royal tombs 48 peat bogs 48 Volcanic eruptions 48 stalagmites 48 earthen walls 48 glacial retreat 47 preserved fossils 47 river deltas 47 species extinctions 47 upwellings 47 melting glacier 47 glaciations 47 mosaic floors 46 temperate forests 46 ancient Babylon 46 Amu Darya 46 soil salinity 46 preindustrial 46 wetland ecosystems 46 Snowball Earth 46 floods droughts 46 hydrologic cycle 46 Khmer empire 46 cholera epidemics 45 worsening droughts 45 torrential storms 45 Siberian permafrost 45 Hetch Hetchy Valley 45 paleoclimate 45 Hittites 45 erupting volcanoes 45 ecological degradation 45 archaeological finds 45 hydrological cycle 45 Qinghai Tibet Plateau 45 lava tubes 45 Judean Desert 45 marine sediments 45 ancient civilizations 45 Tigris rivers 45 Byzantine era 45 Sumerians 45 canals 45 groundwater aquifers 45 Droughts 45 Palaeolithic 45 Mayan civilization 44 cenotes 44 Yuan Dynasty 44 deep gorges 44 underground caverns 44 fourteenth century 44 millennia 44 icecaps 44 freshwater ecosystems 44 classical antiquity 44 ancient tombs 44 Middle Ages 44 aquifers 44 hydro electric dams 44 microfossils 44 Arctic permafrost 44 Severe drought 44 undersea volcanoes 44 Pharaonic 44 Apennine mountains 44 prolonged drought 44 mud volcanoes 44 Bosphorus Strait 44 thawing permafrost 44 Byzantines 44 Roman Empire 44 clathrates 44 rivers 44 ancient relics 44 periodic droughts 44 Euphrates rivers 44 plankton blooms 44 melting permafrost 44 antiquity