When Sergio García-Dils de la Vega kissed his girlfriend, Pilar Orche, goodbye at the entrance to Krubera Cave, he promised to return the next day. But after teammate Bernard Tourte bruised his side in a tight passage, García-Dils decided to stay with him at an underground camp, missing his chance to return to the surface before going deeper. It was two weeks before Orche saw her boyfriend again.

Our expedition, however, had come prepared for a long siege, bringing more than five tons of gear to the cave. Ever since 1956, when explorers in France first descended below 1,000 meters (3,281 feet), generations of cavers had dreamed of achieving the 2,000-meter (6,562-foot) mark. Would Krubera take us there?

Cutting a jagged path through the limestone of the Arabika massif on the edge of the Black Sea, the “trail” to Krubera Cave drops down a chain of pitches, cascades, and pits—some more than 100 meters (328 feet) deep—connected by narrow rift passages called meanders. The cave, located in the separatist region of Abkhazia, was named after Russian geologist Alexander Kruber. In 1960 researchers from the Republic of Georgia explored it to a depth of 90 meters (295 feet). Two decades later, I organized a series of expeditions to investigate new deep caves, using dye traces in cave streams to probe Arabika’s potential depth. In 2001 a team led by Ukrainian Yuri Kasjan set a world record in the cave of 1,710 meters (5,610 feet). Last July a Moscow-based team extended that to 1,775 meters. Our hope was to find a path past 2,000 meters (6,562 feet).

At the start of the expedition, Alexander Karpechenko, whose nickname is “Brick,” exulted in getting his hands on a brand new gasoline-powered hammer that he planned to use to bore holes for explosives to free up tight passages. Team members in nearby Snow Cave cleared blasted rubble from a passage that had been blocked by a “boulder choke.”

Like mountaineers scaling a Himalaya peak, our expedition of 56 cavers from seven countries established a series of campsites at depths of 700, 1,215, 1,410, and 1,640 meters (2,300, 3,990, 4,600, and 5,380 feet). There team members cooked meals, slept five and six to a tent, huddled for warmth, and worked for up to 20 hours at a stretch.

By the third week our downward progress was blocked by a sump at a depth of 1,775 meters (5,823 feet.) Gennadiy Samokhin surfaced after a dive to examine a tight squeeze at the bottom of a ten-meter-deep (32.8-foot-deep) pool. “No chance to get through,” he said.

Searching for a route around the sump, Sergio García-Dils de la Vega braved a cascade of near-freezing water. Also unsuccessful, he discovered to his dismay that his waterproof dry suit had holes in it. “The water was so cold I lost the feeling in my fingers,” he said later. In a last ditch effort, Denis Kurta and Dmitry Fedotov squeezed through a narrow, 100-meter-long (328-foot-long) passage called the Way to the Dream, which successfully bypassed the sump and pointed steeply down. The next day Bernard Tourte and others followed. It was the breakthrough we’d hoped for. The news, spread by telephone to all camps, was greeted with elation, boosting everyone’s spirits.

The newly discovered passage led to yet another sump at a record 1,840 meters (6,037 feet), where Samokhin emerged smiling from a brief test dive. There was a promising downward tunnel, he reported. But it would have to wait. After nearly four weeks of working underground, with supplies running low, our expedition had finally run out of time.

Flush with our success and relieved that team members had incurred only minor injuries, I opened my arms to welcome each of the cavers as he or she emerged back on the surface. Bernard Tourte, his red caving suit, helmet, and gear completely covered with grime, held a congratulatory bouquet of alpine flowers—for a job well done.