It is believed there could be several buildings under the green
Archaeologists are using the appearance of a mysterious hole at Broughty Castle green in Dundee to discover more about what lies underneath the grass.
It is suspected there may be several military buildings below the ground, including pits where mines were armed.
There were plans in the 1880s to put a minefield in the Tay for extra defence.
The archaeologists will also study why the 30cm hole opened up, so the safety of the area can be improved and conservation of the remains considered.
Broughty Castle dates back to 1496 and faced many battles.
In the 1880s, the Tay Division Submarine Miners Royal Engineers set up in Broughty Ferry for a planned deployment of mines in the Tay.
The construction of the castle battery, where the castle green now lies, began in 1903. It was mobilised in 1915 and remained there until the end of the war.
By 1956 all the guns and equipment had been removed from the castle.
The archaeologists have particular interest in the primer pits underneath the green.
Dig leader David Murray said: "They're very carefully built, these thick brick walls, they've got a corrugated iron roof that's designed [so that] in the case of an accidental explosion the blast would be directed upwards."
Dr Kirsty Owen, of Historic Scotland, added: "It's not everyday you get to look at later military remains.
"I think quite often the archaeology of them is neglected, but hopefully we can address that a little bit and add a bit more to the history of the castle."