A guided tour has given the public their first glimpse inside a massive World War II underground fuel depot.
Inchindown oil storage tanks were dug into a hillside to conceal and protect them from enemy attack.
The Royal Commission on Ancient and Historic Monuments (RCAHMS) also used Saturday's event near Invergordon to document the interior of Inchindown.
Allan Kilpatrick, of RCAHMS, who led the tour, said all those who went inside were in awe of its size.
He described the tunnel system as an impressive work of engineering.
More than 40 places were available on the tour organised by Forestry Commission Scotland as part of the Doors Open Days project.
Spaces were booked out within 90 minutes.
RCAHMS plans to host a similar tour on 10 October.
For more than 100 years, the commission has collected, recorded and interpreted information on the architectural, industrial, archaeological and maritime heritage of Scotland.
Photographs taken inside Inchindown are to be added to its records.
Before, during and after both world wars, the deep water port at Invergordon provided an anchorage for the Royal Navy.