Like ancient China, Russia once had a wall to protect itself against invaders—but this wall was green. From the 14th to the 18th century, Russia’s czars kept a thick belt of forest uncut along their southern border to block attacks by Tatar nomads roaming the sparsley forested plains of Central Asia. Within the belt, the Russians dug trenches and felled trees to make the forest impassible to the nomads’ cavalry and wagons. Most of the “Green Wall of Russia” still stands today, ranging from one to 10 kilometers thick. Originally some 375 miles long, 84 percent still remains preserved, centuries after much of the surrounding land has been cleared for farmland.