Jeju International Airport — Jeju City, South Korea

Volcanic in origin and ringed by beaches, Jeju Island depends on CJU as its essential connection to mainland South Korea, and the domestic route linking Jeju to Seoul's Gimpo Airport has for years ranked among the busiest air corridors anywhere in the world by sheer passenger volume. Honeymooners, hikers heading toward the slopes of Hallasan, and beachgoers all funnel through a single, often crowded terminal complex that has struggled at times to keep pace with demand despite periodic renovations and capacity upgrades. International routes bring visitors from China, Japan, and Southeast Asia, drawn by Jeju's UNESCO-recognized volcanic landscapes, lava tubes, and relatively relaxed visa policies compared with the mainland. Land reclamation projects and a long-debated second airport for the island reflect just how tightly capacity constraints have gripped the existing facility over the years. A single main runway handles an enormous volume of takeoffs and landings daily, a testament to how efficiently Korean carriers have scheduled operations around Jeju's outsized role in the country's domestic tourism industry.