Work to re-open a forgotten Victorian tunnel into Carrickfergus Castle is under way.
The 'ground-breaking' start to the excavations of the Harbour end of the tunnel aims to re-establish a entry into the castle.
It will allow access to the castle's Keep by specialist works teams on a roofing project and help ensure the public can continue to get into the castle at this time.
Iain Greenway, director of the Historic Environment Division (HED) at the Department for Communities which is funding the project, said: "It seems right that a castle should have a secret tunnel.
"The tunnel at Carrickfergus is not quite secret but it has been sealed and largely forgotten about for almost 100 years.
"The tunnel was originally an underground railway to move mines and munitions from the Harbour to the Grand Batteries above. Now it will take foot traffic for work on the new roof to the Keep.
"It is an exciting project and - who knows - the excavation may uncover some fascinating artefacts, as well as give us more insight into how the munitions tunnel was originally used."
The tunnel was constructed around 1889, and was used to bring mines and other munitions into the castle complex.
At that time the castle was in military use, as a key defence of Belfast Lough.
The tunnel does not appear to have been used for very long, and was closed when the castle came into state care in 1928.
Anne Donaghy, chief executive of Mid and East Antrim Borough Council, said: "The excitement around this project just keeps building. The castle and its rich heritage offers so much to the town and this development will only reinforce it as a must-see destination. Perfectly situated along the world famous Causeway Coastal Route, Carrickfergus Castle's growth will only continue to benefit both the local people and businesses in the town, and the Northern Ireland economy as a whole.
"This site has incredible potential and we're delighted to be working with the department to uncover all the untapped secrets it holds and hope that people will enjoy the stories this continued regeneration will tell."
Work conducted by HED and its predecessors has confirmed the presence of the tunnel, which is largely intact.
Earlier work has also confirmed that there is a substantial depth of rubble infill material in the passage leading from the floor of the tunnel to an earlier ground level which will require careful excavation to ensure that any earlier buried remains continue to be protected.
Longer-term, work will be conducted to establish if this tunnel may be accessed by the public, once construction is complete.