They are the first humans ever to visit the network in the Mavri Laki region of the island’s White Mountains.
Expedition leader Rob Eavis and co-explorer Robbie Shone – members of the Sheffield University Speleology Society (SUSS) – pioneered the new venture after they found caves in the area were untouched.
In 2005 they began mapping out their journey and after two reconnaissance expeditions, by 2008 they had uncovered 350 individual caves.
The huge volume of crevices, chambers and holes in the ground led them to believe a master chamber existed somewhere below the surface.
After weeks of clambering and squeezing, their belief was confirmed when they discovered a huge underground vault and couldn’t see the floor.
“We thought we found the master master chamber when the team dropped me into a cave and I reached 213m down but had to turn back to catch our flight home,” said Mr Eavis, from Tideswell, Derbs.
“It was a little antagonising feeling like we had found something big but had to leave it because of a booked flight.
“They had lowered me to a quarter of a kilometre under and I was just dangling there looking down into a black void. My torch-light was just disappearing into nothing and it was impossible to see the floor.
“Turning back was very difficult but we decided to return as soon as we could.”
On July 6, at the end of two-week expedition, the group reached the floor of the plunging chamber 553m down. To reach it they even had to navigate the network’s central shaft leading to the surface, itself 100m deep.
The group dubbed the chamber at the bottom ‘Final Destination’ in a nod to the hit movie of the same name. In it teenagers are picked off one by one by unseen character Death.
“To think a group of kids from a university club were the first to charter this place makes us all very proud. The feeling of being the first people there was exhilarating,” said Mr Eavis, 25. “It’s real original untouched exploration.
“There’s nowhere else on the planet where you can do that. The sense of unknown and adventure is staggering.”
Professional photographer and caver Mr Shone, 29, from Litton, Derbs., said: “It’s good to have a certain level of fear because it probably keeps you alive.
“If you were charging around down there without respect then there’s a lot that could go wrong.”
The six team members were all students or former students who joined the SUSS during their degrees. Many stayed on as trainers with the society after leaving.
“One of the most exciting things about going to this valley is that it is probably one of the most cave-dense areas in the world. To find 350 caves in one valley is truly remarkable.
“The university helps us with funding and we record new data for them,” said Mr Eavis. “Plus we get to do what we love best.”