Across
- 2. Combination of different systems or fuels
- 4. Strong building used for defense
- 7. Farming, growing crops and raising animals
- 11. Measures for cleanliness and waste disposal
- 14. Height above sea level
- 15. Roman emperor linked to Aqua Claudia
- 17. Technology using radio waves to detect objects
- 18. Device to slow descent through the air
- 19. First commercial jet airliner
- 20. Supplying water to land or crops
Down
- 1. Aircraft without an engine
- 3. Force that drives something forward
- 5. Faster than the speed of sound
- 6. Irregular air movement causing bumpy flights
- 8. Military observation to gather information
- 9. Organization that protects heritage sites
- 10. Volcanic ash used in Roman concrete
- 12. Basic systems like roads and water supply
- 13. Mixture of metals for strength
- 16. Layer of gases surrounding Earth
- 18. Statement about what is likely to happen