You've probably heard of Iceland's Blue Lagoon – but did you know that this popular tourist destination is completely man-made? Its creation stems from the operation of a nearby geothermal power station, which accidentally created a lagoon of crystal blue geothermal seawater. The idea is pretty simple: After using lava-heated water and steam to turn turbines, the power station feeds the water into the pool. (Don't worry, the water is clean and it's safe to swim.) Locals started bathing in the lagoon back in the seventies and noticed its silica mud had amazing effects on their skin, especially for those who had skin ailments like psoriasis. Today, it's one of the most visited attractions in Iceland.

The lagoon holds six million liters of water and renews every 40 hours. Its on-site spa is routinely named one of the best in the world.

When to go: At the height of the summer solstice in June – when there are few hours of darkness and the sky is often as blue as the water. That said, the water is always warm, and many brave the cold and visit in winter. As a warning, your hair might freeze.

What to know and do: Standard entrance to the lagoon during the winter is 33 euros for adults (about $43), 15 euros for teens 14–15 years-old (about $19), and free for children. During the summer, the fee goes up to 40 euros for adults (about $52). Towels and bathrobes are a few euros extra, but you can bring your own. In addition to bathing in the waters, the site offers incredible saunas and steam baths. Travelers can purchase in-water massages and spa treatments, and if you are looking for a little more privacy, consider the Exclusive Bath and Lounge (330 euros for three hours; about $432) which offers private changing rooms, a lounge with a fireplace, and light refreshments from the on-site restaurant. Before you leave, make sure to check out the skin care line in the lagoon's shop.