Scientists using radar imaging equipment have discovered the ancient passageways at "Gilmerton Cove" extend much further than originally thought.
More chambers have also been uncovered during the research project, BBC Scotland has learned.
The caves lie under the former mining village of Gilmerton in the south of Edinburgh.
Druids and witchcraft
Experts have long been baffled by the origins of the underground passageways and chambers.
They include stone benches, tables and even a small chapel hand-carved out of the sandstone.
The network has been linked to druids, witchcraft, a gentlemen's drinking den and even Mary Queen of Scots.
Now scientists from the universities of St Andrews and Edinburgh are using ground-penetrating radar equipment to map out what else may lie beneath.
The waves bounce off cavities or tunnels carved into the rock below.
The team has discovered that the subterranean network is at least double the size originally thought.
Simon Shackley, from Edinburgh University's School of Geosciences, said: "On the other side of Gilmerton Road there is a rather large chamber that is probably about 4m [13ft] deep."
"There also appear to be cavities in front of the cove and behind it - both about 2m deep".
"Perhaps by the work we're doing - if we're getting a bigger picture we're mapping in a more extensive way the footprint of it - then what we're hoping is that will tell us something about the use," he said.
"If it's got lots more passageways or maybe it's only got these two single passageways, but they might be leading somewhere else. And where they're leading to could potentially tell us what these were used for."
The scientists are now putting together a proposal to undertake more detailed 3D imaging of the site to give them a better picture of the cave network.
Margaretanne Dugan, owner of Rosslyn Tours and a guide at Gilmerton Cove, said: "It's incredibly exciting. We would love it if we could finally find out what lies beyond the blocked tunnels, if we could unlock those secrets and solve the mystery once and for all."
By Fiona Stalker, BBC Scotland reporter