© Photograph: AP
Denis Burakov and Vladimir Shcherbakov set off down the ski slope.
Onlookers watch in horror as giant inflatable ball veers off course and hurtles down ravine at southern mountain resort.

A Russian man has been killed and his friend is seriously injured after their ride down a ski slope in a giant inflatable zorb ball turned to disaster.

A harrowing video captures the victim, 27-year-old Denis Burakov, and his friend Vladimir Shcherbakov, 33, setting off in the ball at a resort in the Caucasus mountains. The ride starts promisingly. But the zorb quickly veers off course, with one man unsuccessfully trying to stop it. It then hurtles leftwards down a ravine. Onlookers watch in horror, one asking: "What's down there?" A voice replies: "Nothing. Catastrophe."

The zorb continued rolling for about half a mile before finally halting near a frozen lake. The emergencies ministry said both men were ejected from the tumbling zorb, and landed on the snow about 10 metres apart.

Rescuers could reach the two tourists only by skiing down a sheer, rocky gully. They dragged the men back up the hill. Burakov died on the way to hospital, from spinal injuries; Shcherbakov suffered concussion, and remains in hospital.

The Russian authorities have opened a criminal case into the accident, which took place in Mount Musa Achitara, a resort in Dombai, in the southern republic of Karachay - Cherkessia. Police are seeking the zorb's owner.

Burakov, a keen snowboarder, and Sherbov, were the first people to try out the ball, which had been set up just an hour earlier next to a beginner's slope, Russian websites reported. Their trip to the resort, on 3 January, was supposed to be a new year's treat.

The accident prompted the head of the national emergency rescue services to demand that Russia tighten its enforcement of safety rules for winter sports, citing a series of accidents over the January holidays.

Sergei Loginov, deputy director of Z-orb.ru, the largest supplier of zorbs in Russia, said the zorbing run that killed Burakov had been conducted in violation of all safety rules. Zorbing required a smooth, gentle slope, with fences on both sides of the track and a secure spot at the bottom where the ball could safely be brought to rest, he said; none of these elements had been present at Dombai.

"It's not even irresponsibility. It's an experiment on life," Loginov said. "It's all or nothing. They either survive or they don't."

The sport of zorbing originated in the 1990s in New Zealand, and is now done around the world.