THE GREAT PYRAMID
It was white once, encased in limestone now removed. One of the earliest pyramids and the greatest: everything downhill from there. Perfectly aligned north south east west, to an accuracy of 0.05 degrees, its position and measurements encode deep geological knowledge of Earth's shape and dimensions. Eerily, its latitude in metres (29.9792458°) is exactly the same as the speed of light in metres per second (299,792,458).
Metres are proportional to Earth's dimensions: initially defined as one ten-millionth of the distance from the North Pole to the equator, later updated to the following definition: the length of the path travelled by light in a vacuum in 1/299,792,458 of a second.