You've seen them your entire life, but how much can you say you know about manhole covers? We uncovered a few little-known facts about these round objects providing access to a world of underground infrastructure.

18 million manholes dot the surface of our country. In the 1st Century A.D., Romans made the first manholes out of sandstone. On older manhole covers, square pegs provided grip for horse hooves. Today’s manhole covers are made of cast iron, concrete or both, and weigh about 110 pounds. The oldest manhole cover in NYC is 150 years old. Manholes are round for safety. A round cover can’t fall down a round hole. A square manhole cover, tilted on it’s side can, risking injury to anyone below. Also, a round manhole cover has a smaller surface than a square one, so less material is needed to cast the cover, meaning less cost to produce. Round manhole covers can be easily transported by rolling, and they don’t need to be rotated to align with the manhole. A popular myth says a nuclear test in the 1950s created enough pressure to jettison a manhole cover out into space.

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