1. Gibraltar Airport, Gibraltar
This airport's 6,000 foot runway is longer than the promontory of Gibraltar is wide (about 4,000) feet. As a result, the final third of the runway is on landfill protruding into the Atlantic Ocean. But the unique feature is that the only road connecting Gibraltar with the rest of Europe crosses the runway at grade. When a flight is taking off or landing, security personnel block the road on either side. Once the flight is off the runway, the gates open up and private vehicles drive straight across.
2. Barra Airport, Outer Hebrides, Scotland
At this airport, planes take-off and land on the beach of the Traigh Mhor. Wooden posts mark where each of the the three runways starts and ends. But when the tide comes in, the runways disappear. British Airways, which runs scheduled service to and from Barra, must therefore schedule flights to correspond with low tides.
3. Princess Juliana Airport, St. Maarten
This airport's runway sits mere feet from Maho Beach. At 7,152 feet, it's barely long enough to handle the widebody jets that fly in from Europe. To ensure enough room for landing, pilots must fly at an extremely low altitude over the beach. As a result, beachgoers are often thrown into the surf thanks to the jet blast they experience at close proximity. Check out this video to see how close the aircraft get to the beach and this one to see what the jet blast can do.
4. Madeira Airport, Funchal, Madeira (Portugal)
When originally constructed, this runway was wedged between tall mountains on one side and the ocean on the other. At only 4,600 feet, it was scarcely long enough for commercial flights from the European mainland. After an TAP Air Portugal 727 sailed off the end of the runway and over a cliff, crashing into the beach, the airport authorities decided to extend the runway. Without much additional land to work with, the airport constructed the runway extension over the water, supporting it with giant pillars around which people can swim or boat at high tide.
5. Lukla Airport, Lukla, Nepal
This airport, nestled among the Himalayas, is the conduit for hikers seeking to scale Mt. Everest. The 1,476-foot runway is sloped downward, with a wall of mountains behind the higher end and a sheer 2,000-foot drop at the lower end. Airplanes take off downhill, and if they can't reach takeoff velocity while on the runway, they drop into the valley and hopefully gain enough lift before they hit bottom. Pilots seeking to land must do so uphill and must keep in mind that the mountains in place do not allow sufficient room for airlines to abort landing and go around. If you aren't unable to land, you're dead.