What's the busiest air route in the world? Heathrow to JFK? Hong Kong to Singapore?
The correct answer will surprise you. The most crowded flight path on Earth, according to a new report by aviation analyst OAG, is actually the 280-mile hop from Seoul Gimpo to Jeju International. A staggering 76,460 flights travelled between the two airports in 2018; by comparison, just over 14,000 flew between London Heathrow and New York's JFK. Seven airlines ply the route: Asiana Airlines, Jeju Airlines, Korean Air, Jin Air, T'way Air, Eastar Jet and Air Busan.
Second on its list is Melbourne-Sydney, with 54,102 departures; third place goes to Mumbai-Delhi, with 45,188. In fact, the 15 busiest air routes are all domestic services.
The busiest international route is Kuala Lumpur-Singapore, with 30,187 departures in 2018, followed by Hong Kong-Taipei with 28,447. Eight carriers run the Kuala Lumpur-Singapore route: AirAsia, Malaysia Airlines, SilkAir, Jetstar Asia, Malindo Airways, Scoot, Singapore Airlines and Ethiopian Airlines.
Europe's busiest service is Barcelona-Madrid, with 18,812 flights last year; Latin America's is Lima-Santiago (10,369).
The most oversubscribed long-haul route is New York JFK to San Francisco, with 15,587 flights in 2018.
Why are so many people flying to Jeju?
Jeju, South Korea Photo: Shutterstock
More than 26 million passengers use Jeju International each year - that's more than any UK airport bar Gatwick and Heathrow.
So what the devil is Jeju, and why are so many rushing to see it?
It's actually the capital of an island, Jejudo, which might just be the most popular holiday destination you've never heard of.
Unesco-listed, and billed as South Korea's answer to Hawaii, it's pure Instagram gold, and home to dramatic volcanic landscapes, underground caves, hiking trails and scenic beaches.
Halla Mountain, at 1,940m above sea level, is South Korea's highest peak, while the cone of Seongsan Ilchulbong, or "Sunrise Peak", is particularly spectacular.
In 2011 Jejudo was named among the "New 7 Wonders of Nature", though there were accusations that its selection was down not just to its beauty, but also the readiness of tourism and marketing organisations to stump up cash – including taxpayers' money – to support its campaign.
There are also casinos, which help lure travellers from China, and, thanks to the island's self-governing status, anyone can visit without a visa.
Bizarrely, there's also a sex-themed park, Jeju Love Land, which features phallus statues, interactive exhibits on the "masterbation cycle", and other sculptures of humans in flagrante. Love Land is said to owe its existence to Jejudo's popularity as a honeymoon destination. Young newlyweds would arrive knowing next to nothing about the birds and the bees so some hotel employees offered to share their expertise. The island soon became an unofficial centre for sex education, making the theme park entirely logical.
But Jejudo also has a dark side. In 1948 and 1949, the South Korean government brutally put down an attempted uprising on the island. Villagers, including women and children, were massacred, and as recently as 2008 mass graves were still being uncovered. Some 30,000 people died as a result of the uprising, with a further 40,000 fleeing to Japan, and the atrocities are remembered at the 4.3 Peace Park memorial.
What do those crowded skies look like?
FlightRadar24 shows planes flying over Australia.
FlightRadar24 tracks every commercial aircraft on the planet and is a wonderful resource for aviation geeks like us. As the image above shows, there are planes flying across most corners of the planet at any given time. For jet-free skies, you'll need to head to Siberia, the northernmost reaches of Canada, the Amazon, the Sahara, the Australian Outback, or Madagascar.
Which is the world's busiest airport?
It's not quite as surprising as Jeju, but not far off. Nope, it's not Heathrow, Beijing or even Dubai. The busiest on the planet is actually Hartsfield-Jackson Atlanta International – as it has been since 1998 (20 years in a row). Almost 107.4 million passengers passing through its terminals in 2018 – that's slightly more than the population of the Philippines.
The biggest airline at Atlanta, by some margin, is Delta. It operates three-quarters of flights. And a remarkable 219 cities have non-stop services to it.
World's 10 busiest air routes:
Jeju-Seoul, South Korea: 79,460
Melbourne-Sydney, Australia: 54,102
Mumbai-Delhi, India: 45,188
Sao Paulo-Rio de Janeiro: 39,747
Hanoi-Ho Chi Minh City: 39,291
Los Angeles-San Francisco: 35,365
World's busiest international air routes
Kuala Lumpur-Singapore: 30,187
Hong Kong-Taipei: 28,447
Hong Kong-Shanghai: 20,678
Jakarta-Kuala Lumpur: 19,741
New York La Guardia-Toronto: 17,038
Hong Kong-Seoul Incheon: 15,770
The Telegraph, London