Blue Lagoon, Iceland

A geothermal spa built on a lava field that charges over $100 for entry and requires booking weeks ahead. The milky-blue water looks otherworldly in photos and like a crowded public pool in person. Iceland's tourism boom — from 300,000 visitors in 2009 to 2.3 million by 2023 — was turbo-charged by Instagram. The country with 370,000 people now gets six tourists for every resident, and the Blue Lagoon is ground zero.