After Pearl Harbor, the U.S. Navy decided to build a radio station in Haiku Valley, in a top-secret plan to enhance military communications. According to the nonprofit Friends of Haiku Stairs, "In order to build the facility a site was needed that had two mountain walls rising as vertically as possible, both a similar height over 2,000 feet, and with flat land in between." Understandably, terrain like that was incredibly tricky to get to. Construction workers put together ladders and affixed them into the mountain with spikes, creating a stairway to access the station.

As dangerous as it was, it was also spellbinding. The vegetation today is beautiful, from a mango tree at the base to palm trees to native plants in the lily family and ferns, and the views of the horizon are unparalleled. It was so jaw-dropping that people started hiking immediately, until the military told them to leave. The Coast Guard eventually opened the stairs to the public in 1975 but closed them in 1987 because of vandalism and safety concerns. Rather than demolish the unsafe stairway, some locals came together to form the aforementioned Friends of Haiku Stairs, which aims to protect the stairs and keep the nearby land clean, in hopes of eventually re-opening the Stairway to Heaven.

Seven decades after they were made, people continue trekking up these stairs, but as time goes on, it becomes more and more risky. In 2015, for example, a storm made the rugged stairway even more difficult to traverse. The Board of Water Supply actually spends $170,000 each year to place guards at the stairs, which are on city-owned land. Trespassing can mean a fine of up to $1,000 and a court date, and in 2016, guards cited more than 300 people. Posters in online hiking communities talk about how the locals are wary of all of these law-breaking hikers. And with fewer hikers than the legal trails, you're less likely to find help if something happens.

But that doesn't stop the Instagrammers, and non-hikers can certainly appreciate the amazing scenery they've captured. Some thrill seekers even took it further, creating a makeshift swing across two rusty poles. The swing was abruptly decried by authorities and hikers alike as unsafe, and it's now been taken down. If you're interested in the sights but not the fines, AllTrails has a page describing what they say is the easiest legal way to hike near the Stairway to Heaven.

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For more sites where you can look but not touch, check out "Forbidden Places: Exploring Our Abandoned Heritage" by Sylvain Margaine. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.