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- World War, 1939-1945104
- Geology38
- Aquifers32
- Groundwater28
- Medicine, Military20
- Water table19
- Floods17
- Hydrogeology14
- Election districts10
- Geology, Structural10
- Physical geography9
- United States. Congress. House9
- Cartography8
- Indigenous peoples of North America8
- Population density8
- Human geography7
- Hydrology7
- Missions7
- Floodplains6
- Geology, Stratigraphic6