It may be well to explain here that it was not possible to begin tunnelling on the same level at which the mine was to be exploded, but considerably more than 150 feet below that level. The tunnel, had, therefore, to be driven on a steep gradient. Another point which the report does not make clear should be borne in mind, viz., that the tunnel divided in the heart of the Castelletto, the main bore being driven on to where the mine was to be exploded, while a smaller branch—referred to below as the “Loop-holed Tunnel”—was run up to a point where favourable exit could be obtained for charging into and occupying the crater of the exploded mine. In all 507 metres of tunnel had to be driven, involving the excavation of 2,200 cubic metres of rock. The details of this work are given in the report as follows:
(A) Chamber for Sullivan Compressor: Dimensions: 5 × 8 metres; average height 2·20 metres.
(B) First part of gallery to second dump of material. Length 72 metres; inclination 38·70 per cent.; elevation gained 25·90 metres.
(C) Second dump of material, established in order to free space for further work and reduce the length of transportation.
(D) Ingersoll Group chamber. Dimensions: 4 × 6·50 metres; average height 2 metres.
(E) Cut from the gallery of the second dump of material to the beginning of the ascent to the mining chamber. Length 136 metres; inclination 4·70 per cent.; elevation gained 6·40 metres.
(F) Ascent to mining chamber. Length 22 metres; inclination 36·30 per cent.; elevation gained 10·75 metres. (This ascent, in order to facilitate tamping, was worked by dividing it into three sections of 1 × 1·60 metres, at nearly right angles.)
(G) Mining chamber. Dimensions: 5 × 5·50 metres; average height 2·30 metres.
(H) Loop-holed tunnel. Length 162 metres; inclination 60 per cent.; elevation gained 83·50 metres in this tunnel itself, or a total of 168·50 from the second dump. This tunnel (the one through which the men were to pass for the attack after the explosion of the mine) had to be strictly confined to the rocky stratum between the Tofana and the Castelletto; its planimetry appears (see map), therefore, rather uneven due to the constant elevation of the rock.