Stand on the step that is exactly one mile above sea level
5,280 feet, disputed
The Colorado State Capitol, gold dome and all, whose west steps carry a marker for exactly one mile above sea level — and whose interior is finished in a pink stone that no longer exists anywhere else on Earth.
Why it's unique
The onyx is the part almost nobody knows. Colorado Rose Onyx was quarried from a single deposit, that deposit was entirely consumed finishing this one building, and no further supply has ever been found. The wainscoting is the world's total remaining stock.
The mile-high marker is the better-known claim and comes with its own quirk: resurveying moved the line more than once, so there is more than one marker and they disagree with each other.
Before you go
Free guided tours run on weekdays and include the climb to the dome — 99 steps up, with the best free view in the city at the top.
Find the disagreeing markers rather than just the famous step; the brass one and the engraved one are not in the same place, and the story of why is better than the photograph.