These declassified photos show the preparations leading up to the nuclear bombings on Hiroshima and Nagasaki. On August 6 and 9, 1945, the atomic bombs were dropped on the two cities. Below are 31 chilling photos of the final preparations:

1. Soldiers checking the casings of the "Fat Man" atomic bomb

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2. Geophysicist Francis Birch (left) marks "Little Boy" L-11 while Norman Ramsey, who would later win the Nobel Prize in Physics, watches. This is the actual bomb dropped on Nagasaki.

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3. Final bomb assembly

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4. Fat Man nearly assembled and about to be placed inside its casing.

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5. A technician applies putty to the forward polar plate of "Fat Man."

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6. Sealant is now being applied using a spray gun.

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8. A frame taken from a colored movie, showing the color of the device and sealant/putty.

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9. Soldiers and workers sign their names and other messages on the "Fat Man."

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10. A closer look.

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11. “A Second Kiss for Hirohito!” signed by Rear Admiral W.R. Purnell, USN on the side of the "Fat Man".

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12. Fully-assembled "Fat Man" is loaded onto a transport trailer.

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13. The bomb is now transported to the airfield, under cover with an escort.

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14. One of two bomb pits on the airfield.

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15. The pits for the bombs, measuring roughly 8 feet by 12 feet, still exist today.

(L) National Archives/ (R) Jeffrey Tripp/FLICKR

16. Soldiers proceed for the preliminary pit alignment.

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18. The bomb's alignment was checked over carefully by eye and hand.

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19. They made sure everything's in place while the MPs kept watch.

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21. Using a hydraulic lift, they carefully lowered the bomb and its trailer into the pit.

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23. Workers check "Little Boy" one last time. Still covered with tarp for security reasons.

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24. Once the bomb is ready, the aircraft Enola Gay, a Boeing B-29 Superfortress bomber, is backed up slowly over the pit.

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25. The cover is removed and the bomb is readied for loading.

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27. "Little Boy" is carefully raised using a hydraulic lift and loaded into the belly of the Enola Gay.

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30. Once the bomb is secured inside the plane. All connections and equipment are checked again.

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31. From there, "Little Boy" and "Fat Man" were flown over Hiroshima and Nagasaki, respectively, and detonated. World War II ended shortly afterwards.

National Archives / Charles Levy / AP