Thousands of years ago, ancient cultures across Europe built massive stone structures with extraordinary precision. Scottish engineer Alexander Thom discovered these sites all used the same exact unit of measurement: 2.72 feet, which he called the Megalithic Yard. This measurement appeared everywhere, from Stonehenge to the Scottish Isles, at sites separated by thousands of miles and built across thousands of years. These structures weren't just monuments - they were sophisticated machines that tracked celestial movements with astonishing accuracy. How did ancient people without writing or advanced tools achieve such mathematical precision? Why does this measurement connect to fundamental constants of the universe? And most puzzling: why did they suddenly stop building these structures?