natural or manmade

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Across
  1. 3. a place where planes land and take-off.
  2. 6. a manmade structure for crossing a river.
  3. 8. a manmade structure for passing under a road or a river.
  4. 9. a tall, narrow building, often in a castle or church.
  5. 10. a very tall building, usually in a big city.
  6. 11. low land between mountains.
Down
  1. 1. like a a hill, but taller.
  2. 2. like a mountain or a hill, but with a crater at the top for gas or lava to escape.
  3. 4. coral just under the surface of the sea.
  4. 5. water falling from a height.
  5. 7. a piece of land surrounded by water.
  6. 12. water with land all around.