Hoover Dam Geodetic Survey Markers

The Hoover Dam is one of America's greatest feats of civil engineering, capable of holding back a staggering 10 trillion gallons of water. Of course, a project of this kind had to be made with precision, and even after its completion, that preciseness must be maintained. 

This job is done during a geodetic survey where the exact location and elevation of certain points on and around the dam are measured. These locations are then marked with benchmarks to allow future surveyors to see how things changed over time. 

The area around the Hoover Dam is surveyed every few decades where all markers are re-measured, and new ones are added where needed. This is how it was discovered that the dam subsided by just under five inches between 1935 and 1964. (Don't worry this was within the predicted values.)