The largest island in the world is Greenland. Probably. You have to disqualify all the bigger landmasses, from Australia on up, as continents, and ignore the possibility that Greenland might actually be three smaller islands connected by an ice sheet. But what about the world’s smallest island? Would every single pebble in every single mud puddle theoretically be a contender? What makes a real island?

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The British government actually answered this question in 1861, officially designating as an island any dot of land that either (a) is inhabited, or (b) has enough grass for “the summer’s pasturage of at least one sheep.” (About two acres, if you don’t have sheep appetite numbers handy.)

In that spirit, the _Guinness Book of World Records _gave its “Smallest Island” award to Bishop Rock, the westernmost of the Isles of Scilly, which lie off Britain’s southwestern tip, the peninsula of Cornwall. Bishop Rock is much too small to pasture anything: at low tide, it’s a rocky ledge 46 meters long and 16 meters long, about the size of three tennis courts.

In the Middle Ages, Bishop Rock was known as Men Epskop, and criminals were rowed out there with a few days’ worth of bread and water and abandoned to the waves. But in 1847, after a series of costly shipwrecks in the area, the Crown decided to erect a 167-foot lighthouse on the island, which still stands today. At high tide, the lighthouse covers every square inch of Bishop Rock, so you can be sure that this project was an engineering nightmare. What are the workers supposed to even stand on?

After a decade of very tricky construction—and after one almost-completed lighthouse blew over in a storm—the Bishop Rock lighthouse was completed. The keepers moved into the place in 1858, meaning that Bishop Rock was inhabited, and therefore Britain’s smallest island. (The light’s been automated since 1992, so Guinness might be technically wrong about Bishop Rock—though they still insist it’s the “World’s Smallest Island with a Building on It"). If you have access to a tiny sandbar somewhere, build a little shed on it. You’ll be famous.