A Ukrainian team has set a new depth record for caving.
The nine-strong group travelled 2,080m (6,822ft) underground, passing the elusive 2,000m mark at Krubera, the world's deepest known cave.
The team was part of a project that has made breaking through 2,000m its goal for the past four years.
It built on records set by a previous expedition, which blasted through blocked passages in the cave, within Georgia's breakaway region of Abkhazia.
"Even now, we don't know whether we've reached the limit - or if it will go on. We're pretty sure we'll eventually go even lower," said Alexander Klimchouk, the veteran caver who organised the mission.
DEEPEST KNOWN CAVES
Krubera, Georgia (Abkhazia) 2,080m (6,822ft)
Lamprechtsofen, Austria 1,631m (5,354 ft)
Gouffre Mirolda, France 1,626m (5,335 ft)
Reseau Jean Bernard, France 1,602m (5,256 ft)
Torca del Cerro, Spain 1,589m (5,213 ft)
Sarma, Georgia (Abkhazia) 1,542m (5,062 ft)
Cehi 2, Slovenia, 1,533m (5,030 ft)
The Ukrainian Speleological Association's Call of the Abyss project is funded by the US National Geographic Society.
During an expedition from August to September 2004, a team of 56 cavers (45 men and 11 women) representing seven countries explored Kubera, deep below the Arabika mountain massif of the western Caucasus.
Carrying about five tonnes of equipment, they had to negotiate vertical drops and freezing torrents of water. They were also forced to blast rubble from passages that were critically narrowed or blocked by "boulder chokes".
They set camps at depths of 700m, 1,215m, 1,410m and 1,640m, where they cooked meals, slept up to six people to a tent and worked for up to 20 hours at a stretch.
Team members had to negotiate cold pools of water (Image: National Geographic)
The cavers kept in touch with the surface base camp by rigging nearly 3km (two miles) of rope strung with a telephone wire.
But the August-September expedition encountered many obstacles. By the third week, a sump (a flooded passage) blocked the team's downward progress.
When team member Sergio Garcia-Dils de la Vega investigated if there was a way through, he was confronted by a cascade of near-freezing water and was forced to retreat after discovering his waterproof dry suit had holes in it.
Finally, colleagues Denis Kurta and Dmitry Fedotov squeezed through a narrow, 100m-long passage, which successfully bypassed the sump and pointed steeply down.
In October, a team of nine cavers was sent back to Krubera to pick up where the previous group left off.
The August-September mission paved the way for October's record (Image: National Geographic)
They examined all unexplored leads in the cave's lowest section until they broke through to a new series of passages and vertical pits. On 19 October 2004, team leader Yuri Kasjan dropped down a pit and discovered from his altimeter that he had passed 2,000m.
More pits and passages brought the explorers to a sandy chamber at 2,080m, the deepest to date any caver has ventured below ground (gold miners in South Africa regularly go beyond 3,400m).
The team christened the chamber Game Over. But the group now wants to return to the cave to see whether it leads even deeper.
Details and photographs of the expeditions appear in this month's National Geographic magazine.