I'm still trying to figure out how long it takes to make eight million pounds of iron into the form you want and attach it all together. Can you imagine the cranes they must have used to get all the parts into place? To give you some idea how much iron that is, the Eiffel Tower weighs twenty million, two hundred thousand pounds overall, and stood 852 feet tall when completed in 1889. There's more goodies on the dome here:
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