Unless you know where to look, Playa del Amor (or "Beach of Love") in Mexico's Marieta Islands is hard to find. It's concealed inside a sinkhole cave, and the only way in is by swimming or kayaking through a water tunnel. Though it looks like a natural marvel, the cave around the hidden beach may have been created by human activity within the last century.
According to Atlas Obscura, Mexico started using the uninhabited Marieta Islands off the coast of the Riviera Nayarit to test bombs in the early 1900s. The controlled explosions changed the landscape, creating caves, craters, and most likely the sinkhole above Playa del Amor.
The practice continued for decades, and in the 1960s, scientist Jacques Cousteau launched a campaign to conserve the island chain's natural resources. The Mexican government made the Marieta Islands a national park in response to the protests. Later, the site was declared a UNESCO MAB Biosphere Reserve.
Parque Nacional Islas Marietas is a protected area today, but tourists are allowed to visit on a limited basis. After traveling through the spot's one narrow entryway, visitors are allowed to sunbathe beneath the circular patch of sky and swim in the clear water. Thanks to conversation efforts, the beach is also a great place to see wildlife that's returned to the area.
[h/t Atlas Obscura]