Caves are fascinating.

But many people are unaware of one danger: small unventilated caves can present gas chambers which fill in time with extremely toxic gases like carbon dioxide, carbon monoxide, sulfur dioxide, ammonia, methane and others.

Yesterday, a maze of narrow underground caves in Tenerife Island, from Canary Archipelago (Spain), off northwest Africa, locally known as Piedra de los Cochinos (the Stone of the Pigs) made 6 victims out of a group of 30 scientists and nature lovers. "The explorers were more than a mile underground Saturday in area where gases may have seeped in, cutting off their oxygen," said Jose Andres Garcia, the island's emergency services director. "The six likely died when they inhaled air filled with carbon dioxide", said Eustaquio Villalba, a spokesman for the Tenerife Friends of Nature Association. "It doesn't smell bad or of gas and causes a depletion of what little oxygen is available down there, given there is no ventilation," he said.

Other gases - like sulfur dioxide and ammonia - stink (in these cases, like rotten eggs and urine, respectively). "One person who managed to make it out alerted emergency services," said Jose Miguel Ruano, Tenerife's regional government minister.

The rescue operation of the 5 bodies of men and one woman took 17 hours; six others victims, still alive, were flown by helicopter to a hospital. "Rescue efforts were complicated by the gases and because the tunnels are so cramped," Ruano said.

Some tunnels are centuries old and have few stairs or lights and the zone were the corpses were found was just three feet (1 m) in diameter. "Officials discourage exploration of the tunnels, carved out to extract water in the volcanic island off the west coast of Africa," said Jose Segura, an Interior Ministry official.

With all the measures, adventurers are attracted by the beauty of these caves, and force the steel gates as policing the tunnels is impossible. "The group probably got lost because they ventured in without their guide, who had to cancel and give them instructions by cell phone before their descent," said Villalba. "Her Spanish companion was hospitalized. The couple found a stray dog that might have helped them find their way out, he told the Finnish news agency STT", said Heikki Viironen, the father of a Finnish woman who escaped unharmed, about his daughter's experience. "We thought perhaps the dog's sense of direction saved them," Viironen said. "Among the group were scientists from a renowned astrophysics observatory on Tenerife's dormant Teide volcano," Ruano said (including the Finnish woman).

Many of the group belonged to the Tenerife Friends of Nature Association. "An Italian was among the dead," the Italian foreign ministry said. "In some parts of the tunnels, humidity can reach 100 % and temperatures can reach 86 degrees. However, it can be spectacular, with underground waterfalls visible," said Villalba.