2018:910 - Carrickfergus Castle Munitions Tunnel, Antrim

This project involved a four-week excavation of the 1889 munitions tunnel at Carrickfergus Castle. It is intended to create here a secondary entry point to the interior of the castle and enable future works to allow access to the castle with minimum interference to the visitor flow. Specifically, the works lay within the southern half of the Inner Ward of the castle and ran from the external wall of the 1880's stores on the eastern side of the Inner Ward westwards, via a brick-vaulted east to west downwardly inclined tunnel, to the steel doors on the outer face of the castle crag beside the harbour. This tunnel was constructed by the Royal Engineers in 1889 as part of the minefield defences of Belfast Lough, designed to protect the heavy industry and shipyards of Belfast. Narrow gauge rails ran up this tunnel from the harbour into the castle Inner Ward.

The east side of the tunnel (upon exiting the vaulted section) gave way to a passage which became more shallow to the east where it levelled to meet a turntable. The rotating bridge of the turntable was no longer present, but the substructure was still in place. The brick substructure within the passage and associated granite coping was exposed and recorded. Further rails extended north and east from the turntable, the former gently inclined, and led towards the Middle and Outer Wards. The latter was not followed, but would have led to another turntable with rail access to a mine storage facility along the east wall of the inner ward. Some of this had been previously investigated by Queen's University CAF. The overall length of the tunnel from harbour gates to turntable was 21.5m, with a turntable diameter of approximately 1.4m. From the turntable the rail was exposed for a distance of 0.5m eastwards and 4m northwards. The rail gauge was 0.5m or 19½ inches and the rail line comprised two parallel wrought iron or steel rails attached to iron or steel sleepers.

On either side of the open passage of the tunnel the surface was investigated to a total trench width of 5m. This exposed a concrete foundation, probably for a crane associated with the tunnel and passage to manipulate the mines, several earlier yard or floor surfaces (including a cobbled surface) which were truncated by the construction of the tunnel, the remains of a surface post-dating the tunnel construction and a later brick surface.

The munitions tunnel was constructed through several earlier deposits. The most significant of these was a layer of cobbles which were part of a pre-19th-century floor, or yard, and the basal layer which appeared to be the original ground surface prior to the construction of the castle. This layer contained worked flint artefacts which dated from the Mesolithic through to the Bronze Age and indicates prehistoric occupation of the promontory on which the castle sits.

This rail system was part of a Submarine Mining station which was the lynchpin of the state of the art Late 19th-century defences of Belfast Lough designed to protect the shipyards and heavy industry form surface raiders, supported with gun batteries and searchlights. Around thirty similar systems were constructed at naval bases, fuelling stations and industrial and mercantile hubs around the British Empire. The system was developed in the late 1860s with the advent of reliable electrics and, although was only operational in Belfast Lough for around five years, provided a relatively inexpensive defence for valuable assets and avoided the necessity of having large sections of the Royal Navy being tied up in ports defending land-based assets. The death knell of the submarine mining system was sounded by two things: workable submarines with accurate torpedoes and HMS Dreadnought-style, big-gun battleships with long range guns that could sit ten miles from the target and pound it to pieces.

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