Sesena, Spain

Sesena in the background behind an illegal tire dump site. (AP Photo/Daniel Ochoa de Olza)

These disturbing photos may make you want to trade in your car for a pair of good walking shoes.

On a scrubby hillside outside of Madrid in central Spain, an inky black mass blights the landscape. A closer look reveals millions of discarded tires — and an environmental nightmare waiting to happen, according to Phys.org.

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Since the 1990s, more than 75,000 tons of car tires have piled up in a landfill inSeseña, a sparsely populated suburb 20 miles south of Madrid, CBS New reports.

The tires were originally supposed to be recycled, but started to pile up when the company operating the site, Disfilt SA, failed to dispose of them, according to Spanish-language news site El Pais. Now, the mountain of rubber spans a whopping 25 acres."Tires were being brought here but no tires were leaving, so a dump developed,” Seseñamayor Carlos Velazquez told Phys.org.

In 2003, the site was declared illegal, but Disfilt SAfled after the owner was sentenced to prison and the company was slapped with steep fines for environmental crimes. Since then, the arduous task of recycling the tires has been left to the local government.

It doesn’t help thatSeseñaitself is somewhat of a controversy: Sprawling developments built during Spain’s housing boom never drew many buyers, leaving the city looking more like a ghost town, according to The New York Times.

And the tire dump is a huge environmental hazard that could further deter future growth. Already, residentslive in constant fear that the heap of rubber might catch on fire, according to Yahoo.

"A fire would be very difficult to put out because tire combustion is very slow,"Vicente Garcia deParedes, an activist with the environmental group Ecologists in Action, told El Pais. "Besides cutting off air traffic, the smoke would be so toxic that the 11,000 homes in El Quiñón de Seseña would have to be evacuated."

Aside from the fire risk, discarded tires leach oil and chemicals into groundwater and the stagnant water they collect becomes a breeding grounds for pests, according to The New York Times.

The town now has a three-year plan to grind down and truck out the tires to recycling plants, according to Phys.org. The Seseña tires will likely find a second life as building materials: the floor mats, rubberized asphalt and rubber bumpers that surface roads, sports tracks and kids' playgrounds.

But until the tires are cleared, Seseña has an unwanted neighbor that could keep the town from ever reaching its intended potential."The visual impact is dreadful. The countryside is totally spoiled by this black mark," Garcia de Paredes told Phys.org. "This is an area of great views. Drivers on the highway to Andalucia will have this view stuck in their minds."