Since the 1960s, a number of tunnel-like structures unearthed at various locations within the walled city, eg East Wall, Shipquay Street, the Diamond, Bishop Street and St Columb's Cathedral, have fuelled speculation about the existence of underground passages. However, conclusive proof that these discoveries are part of an overall network of tunnels traversing the city remains elusive.
The East Wall find, uncovered on Christmas Eve 1967 during renovation work at the old YMCA building, led one local architect to conclude that the quality of the workmanship of the vaultlike structure indicated that it could be part of a planned system of passages constructed before 1620. He commented:

It does indeed seem highly probable to many that in a walled city overlooked by higher ground advantageous to potential invaders underground passages would exist. Curiously, however, nowhere among the records concerning the planning and financing of the original construction and later improvements of the Walls is there any explicit mention of tunnels or underground passages, an obvi-ously major undertaking meriting at least some acknowledgement. Notwithstanding this, tunnel protagonists identify an item listed in a 'return of expenditure' document as an implicit reference to the building of interconnected passages. It reads:

Tunnel sceptics, on the other hand, argue that underground features examined as part of the 'Survey for Tunnels in the Walled City' – supervised by the then Department of Architecture and Town Planning in July 1971 – show indisputably that the reference plainly related to vaults and cellars for storage.
After the completion of the survey, the British Army sealed up entrances to the underground structures and allegedly confiscated a 'chart of the tunnels' purportedly giving exact details of the entrances and passageways.
It is possible that other 'evidence' has been destroyed by accident or building work over recent years. The debate continues and will only be decided if a thorough and comprehensive research and excavation programme is permitted to finally solve the mystery of Derry's tunnels. During the 1970s most of the Walls were carefully restored and large sections exposed to the public view for the first time in centuries.
Today the Walls are still owned by the Irish Society but are administered and maintained by the Historic Monuments Branch of the Department of Environment and Derry City Council, which attempt to preserve the stonework on an ongoing basis.
Little did the original builders think that almost four centuries later the Walls would still be called upon to play a part in security operations as sections were still closed to the public in 1994. They were finally re-opened not long after when the Peace Process finally took hold.
The full circuit of Derry’s Walls is now open to resident and visitor alike and hopefully will remain so, as walking their magnificent ramparts is the only way to capture the feel of Derry's unique Walls and journey briefly into its momentous history.