Walking across a line between two high entities isn't a new feat. But while traditional tightwires are run with high tension, walking a slackline is arguably more challenging.

The nascent sport requires a more dynamic set up: A flat nylon webbing is used for the line, allowing for greater movement and oscillations.

For an even greater challenge, slackliners set up "highlines.” To walk a highline requires fiendish mental control as one balances over a highly exposed abyss, contending with the flux of natural elements.

With evermore outlandish lines being set, the sport is constantly being pushed to the extremes, as depicted in this collection of the most gnarly lines out there.

The Big Wave Line, Nazaré, Portugal

It's not just the big-wave riders that are the daredevils at Nazaré. Slacklining group Western Riders set up a 98-foot line, stretching from the iconic lighthouse at Nazaré to Guilhim rock.

But crossing the line, being pounded by spray from giant waves and gusting winds isn't a walk in the park.

As slackliner Emerson Machado, who braved the line explains, "even after years doing highline, it's still scary. The exposure, the height, wind and weather conditions can mess with your mind."

The Hot Air Balloon Line, Barcelona, Spain

Fly two hot air balloons to 5,905 feet above sea level. Stretch a 49-foot line between the two balloons. Add in three ballsy slackliners, take away the safety leash and you've got yourself the closest thing to skylining (or insanity) out there.

"To walk a line, in such an altitude, which you have never tried before, without a leash with everyone staring at you is hard," describes slackliner Niklas Winter. "On top of that, the balloons were moving quite a lot."

The slackliners only had one shot at making it across. Step a toe out of line, and they plummeted to the ground, relying on their parachutes to land safely.

The Cave Line, France

It's not all about big blue skies. The Sangle Dessus Dessous (SDD) slackline group and a team of cavers shimmied down into the French cave Gouffre Berger (once thought to be the deepest cave in the world at 3,680 feet deep) to take 'deeplining' to a new level. Their 262-foot line was strung up 1,640 feet below the ground.

"It's a pretty strange feeling to be high above the ground in a cave," notes slackliner Antony Newton. "The perception of height and time is different, and you can feel the immensity of the place weighing on your shoulders."

The Eiffel Tower Line, France

Antony Newton walking his way into the urban highline world record. Photo: Courtesy of SDD Slackline

Antony Newton walking his way into the urban highline world record. Photo: Courtesy of SDD Slackline

Walking this 2,215-foot line set a new urban highline world record, and inspired an insatiable hunt for more aesthetic lines. Guillaume Rolland from SDD notes, "It's usually obvious gaps (or buildings) that are just waiting to be connected, with the picture of a slackliner walking between them enhancing the beauty of the scene."

Yet, often the trickiest part is not walking the line, but rather completing the "hard task of asking for permits," says Rolland.

The Kilimanjaro Line, Tanzania

Swiss Alpinist, Stephan Siegrist set a a new world record by walking across a highline at 18,700 ft between the rock towers above Arrow Glacier Camp, Kilimanjaro.

The cold makes it tricky to get into the relaxation state necessary for a good performance, and Siegrist adds: "at this altitude, everything is slower – and that goes for balance too."

The Blindfolded Line, Dali, China

Adding an extra slice of spice, Signoret walked this line while blindfolded. Photo: Courtesy of @sablopignoret

Just to keep things interesting, Pablo Signoret set another world record with this gnarly challenge, walking a highline of 1,410 feet while blindfolded.

Signoret explains, "taking away your sight is challenging but once you get used to it, it becomes the most intense feeling you've ever had on the line."

The World's Longest Line, Cirque de Navacelles, France

A line so lengthy, it curves. Photo: Courtesy of @sablopignoret

In June 2017, Pablo Signoret, Nathan Paulin and Lucas Millard successfully trotted across a 5,452-foot line that dangled 1,115 feet off the ground. They now hold the current world record.

Although highlines increase the mental challenge for the athletes, Signoret stresses one should remember to "enjoy the view and smile!"

The Gondola Line, Switzerland

Not a sight one often sees from the gondola. Photo: Courtesy of @newton_slack

When bundled into a cable car, spotting a person tentatively tiptoeing along a thin line alongside is rather unnerving. Even more so when he's 328 feet off the ground and just 10 feet away.

The gondola passengers had "the best view" remembers slackliner Antony Newton, who boldly completed this feat.

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