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.
9 Facts about Manhole Covers
- 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.