What's the busiest air route in the world? Sydney to Melbourne? New York to Washington DC? London to Paris?
The correct answer will surprise you. The most in-demand flight on Earth is actually the 450km hop from Seoul Gimpo International (stop sniggering) to Jeju International. More than 11 million journeys were made between the two South Korean airports in 2015, the International Air Transport Association (IATA) reports, and with a one-way capacity of 6,561,314 for 2016, according to aviation number crunchers OAG, that figure could yet rise to 13m this year.
Second on IATA's list is Sapporo-Tokyo Haneda, with 7.8m. But the Sydney-Melbourne route's reputation as one of the world's busiest is well deserved, coming in at fourth place. That's a high ranking considering the comparative sizes of the Australian cities to other destinations on the list.
New York-Washington DC and London-Paris, meanwhile, don't even make the top 10. The latter isn't all that surprising - most travellers between London and Paris now take the Eurostar train.
The world's busiest air routes
- Jeju International - Seoul Gimpo International - one-way capacity (2016) - 6,561,314
- Sapporo New Chitose - Tokyo Haneda - 6,209,366
- Fukuoka - Tokyo Haneda - 5,961,277
- Melbourne - Sydney Kingsford Smith - 5,067,167
- Taipei Taiwan Taoyuan International - Hong Kong International - 4,146,547
- Delhi - Mumbai - 4,143,639
- Ho Chi Minh City - Hanoi - 4,141,322
- Beijing Capital International - Shanghai Hongqiao International - 3,962,081
- Surabaya - Jakarta Soekarno-Hatta - 3,849,866
- Tokyo Haneda - Okinawa Naha - 3,784,546
A quick search on Skyscanner shows the planes bound for Jeju depart Seoul Gimpo with unerring regularity. Fancy going there on Monday? You could catch the 1935 service, the 1945, the 2010, the 2025, the 2045…
More than 26 million passengers used Jeju International, though this is significantly fewer than Australia's busiest - Sydney Airport's 42 million.
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.
The UNESCO World-Heritage-listed Manjanggul Cave in Jeju. Photo: Getty Images
Halla Mountain, at 1940m 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 "New7Wonders of Nature", though there were accusations that its selection was down not just to its beauty, but also the readiness of tourism or marketing organisations to stump up cash – including taxpayers' money – to support its campaign.
See also: Meet the tough women hunters of Jeju
There are also casinos, which help lure tourists from China, and, thanks to the island's self-governing status, anyone can visit without a visa.
One of Jeju's major attractions: the theme park Love Land. Photo: Getty Images
Bizarrely, there's also a sex-themed park, Jeju Love Land, which features phallus statues, interactive exhibits on the "masturbation 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.
The Telegraph, London