During the seige of the castle by the Earl of Arran in 1546 - 1547. The French / Catholic attackers dug a seige mine in an attempt to (literally) undermine the foundations of the castle. The idea was to remove a large quantity of supporting rock from under the foundation, while supporting the roof of the mine with timber. When sufficient rock had been removed, the timbers would be set on fire and thus become unable to support the weight of the wall and foundations. This was very hard and dangerous work, especially in St. Andrews where the mining was done by pick axe through solid rock.

The technique for this type of seige was well known. There having been a good example of its use in the seige of Rochester castle in the early 13th century.

The defenders' counter-mine (upper left) was small and cramped. The attackers' mine (centre and right) was roomier. Illustration: Historic Scotland

The defenders of St. Andrews castle were well aware of the mining attempt, and dug a counter-mine to try to intercept the attacking mine. Guided by the sound of the attackers digging, and racing for time against the attack, their counter mine was much smaller and branched off in different erroneous directions. Eventually the two mines met at the top of the ladder in the picture above.

The mine was re-discovered in 1879 when the foundations for a house, across the street from the castle, were being installed.

The entrance to the mine is in the foreground, the house where the mine was discovered while building foundations, is in the background.

Entrance to the mine is via the counter-mine from within the castle grounds. The fact that the entrance is outside the walls of the castle, gives credence to the theory of there being an outer defensive position on the castle side of the defensive (dry) moat.