Three Mile Island Is Only 2.5 Miles Long

If you thought the name "Three Mile Island" referred to its actual length, you'd be wrong. The infamous Pennsylvania island in the Susquehanna River is only about 2.2 miles long at its maximum—roughly half a mile shorter than its name suggests. The irony? The name itself was a mistake from the start.

In 1963, the U.S. Department of the Interior conducted a geological survey based on aerial photography from 1961. Someone eyeballing those images apparently believed the island stretched about three miles, and the name stuck. But they were off by nearly 0.8 miles—a significant miscalculation for something that became one of the most recognized island names in American history.

A History of Name Changes

Before its infamous moniker took hold, the island went by several other names depending on who owned it. In the mid-1800s, it was Musser's Island. Later it became Conewago Island, then Elliot's Island, and eventually Duffy's Island. Each name reflected the property owner of the time, as was common for river islands used for farming and industry.

There's also an alternative theory about the "Three Mile" designation: the island sits approximately three miles south of Middletown, Pennsylvania. Some historians suggest cartographers may have used this distance as the naming reference, especially since this stretch of the waterway unusually lacked locks for about three miles.

From Obscurity to Infamy

The island's quirky name might have remained a footnote in Pennsylvania geography if not for the events of March 28, 1979. That's when a partial nuclear meltdown at the Three Mile Island Nuclear Generating Station turned this small river island into a household name worldwide. The accident became the most significant in U.S. commercial nuclear power plant history.

The nuclear facility, built in the 1970s, occupied the northern tip of the island. Unit 2 suffered the meltdown, while Unit 1 continued operating until 2019. Interestingly, in 2024, Microsoft announced plans to help restart Unit 1 to power its data centers—bringing the island back into headlines once again.

So next time someone mentions Three Mile Island, you can share this geographical quirk: the island is not only famous for a nuclear accident, but it's also named after a length it never actually was. It's a double layer of historical irony wrapped around a 2.2-mile-long piece of Pennsylvania real estate.