TOOMEY: Welcome back to Living On Earth. I’m Diane Toomey.

Once upon a time, a great wall was built to divide a sprawling empire from the barbarian tribes to the North. If you’re thinking the Great Wall of China, think again. 1,880 years ago the Roman emperor Hadrian ordered construction of the wall that bears his name. Hadrian worried that his empire was getting too large to defend, so he ordered the borders reinforced.

Where possible, boundaries were set at natural features. But when Hadrian visited Britain in 122, he saw no obvious demarcation. So the Romans chose one of the narrowest sections of the island near the present day Scottish border, and built a thick stone wall, 15 feet high, stretching 73 miles from coast to coast.

Among the artifacts discovered in the vicinity of the wall are messages written in ink on small wooden tablets. "I have bought 5,000 bushels of grain and unless you send me some money, I shall lose my deposit and be embarrassed," wrote a man named Octavius on one of the tablets.

Over the centuries the wall was treated like a quarry, and its stones carted off to build farmhouses, field walls and churches. Today, remnants of the ancient wall still snake across Northumberland, and more than a million people visit this World Heritage Site every year.

And for this week, that’s the Living On Earth Almanac.