When planning began on the World Trade Center towers, they were only supposed to be 100 ft. taller than the 1,250-ft. Empire State Building, which was still the world’s tallest at that point. But on Dec. 23, 1970—45 years ago—when the North Tower finally “topped out” at its highest height, it had gained even a few more feet on that estimate, reaching 1,368 ft. by the Port Authority’s count.

But the noteworthy nature of the towers wasn’t just a matter of friendly competition among city landmarks. These construction photos make clear that they were a feat of engineering. Viewed in hindsight, of course, it is not just impressive but also bittersweet to see the birth of a building that would meet such a tragic end.

Today, the One World Trade Center tower measures 1,776 ft.

Read original coverage of the plan to build the World Trade Center, here in the TIME Vault: Onward & Upward

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Write to Lily Rothman at lily.rothman@time.com.

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