For a period of time back in 2007, "ghost flights" with no passengers onboard plagued London's Heathrow Airport and caused an uproar among environmentalists, as Jalopnik reports.

Six times a week, the now-defunct British Mediterranean Airways sent planes that were practically empty—save for crew members—from London to Cardiff, Wales, and back again the next day. And it all had to do with Heathrow's strict policing of runway rights, as YouTube channel Half as Interesting explains in the video below.

Despite being the UK's largest and busiest airport, Heathrow only has two runways. For comparison, Chicago O’Hare International Airport, which sees roughly the same number of passengers, has seven. As a result, the 650 daily "slot pairs" that grant airlines the right to take off and land at Heathrow are costly and hard to come by. They also tend to command sky-high prices when they're traded and sold. Oman Air, for instance, paid $75 million for a prime early morning slot in 2016.

Airlines have to use their slot at least 80 percent of the time over a six-month period, lest it be forfeited to another airline. So when British Mediterranean’s service from London to Tashkent, Uzbekistan, was abruptly canceled, it scrambled to ensure it wouldn’t lose its slot at Heathrow. Hence the ghost flights.

Environmentalists, of course, were less than pleased with the fact that each 140-mile flight resulted in five tons of CO2 being released into the atmosphere, according to the BBC. The ghost flights eventually ended and British Mediterranean Airways was sold after suffering a series of losses. As Half as Interesting notes, although extreme cases like these aren't as common anymore, some airlines schedule cheaper flights more often in the winter to ensure they’re making full use of their slot.

And Heathrow is once again taking some heat from environmentalists. The airport is planning to expand by adding a third runway, which sparked protests outside the Houses of Parliament last month.

[h/t Jalopnik]