For a perfect square: Use the circled letters to see what to find this in the backyard

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Across
  1. 5. AKA also known as Aqua Lung
  2. 9. Manufacturer of the F-35
  3. 10. Jacques's sea adventuring ship
  4. 11. Maker of fastest RC car XO-1
  5. 12. Tallest LEGO set
  6. 13. provides thrust without a jet
  7. 15. Where to find the Nether
  8. 16. Early oceanographer and co-inventor of Aqua-lung
Down
  1. 1. White wizard who betrayed Gandalf
  2. 2. Ratio to speed of sound
  3. 3. Author of Lord of the Rings trilogy
  4. 4. Deepest ocean trench
  5. 6. Red and white peppermint stick
  6. 7. Largest ship at 458 meters long
  7. 8. Early brothers of flight
  8. 14. Ringbearer from the Shire