The Men Who Tunnelled Under a Mountain

By the mid-1940s, Belfast was growing faster than its water supply could keep up. Silent Valley reservoir, completed in 1933 after a decade of construction and the loss of nine lives, was already reaching capacity. The Belfast Water Commissioners needed more water. They found it in the Annalong valley — on the far side of Beann Each (Peak of the Horses), the mountain known today as Slieve Binnian.

The problem was simple and enormous at the same time. The water was on the wrong side of the mountain. To reach it, someone would have to go through 2.5 miles of solid Mourne granite — underneath one of the highest peaks in the range.

In 1947, over 150 men began the work. They drilled and blasted their way through the granite heart of the mountain, advancing metre by metre in conditions that would make most people turn back at the entrance. The air was thick with dust. The noise of the drills echoed through solid rock. They worked in shifts, underground, in near-total darkness, driving the tunnel from both ends toward a meeting point deep beneath the summit.

The tunnel linked the Annalong valley water catchment to the Silent Valley system, effectively doubling the reservoir’s supply. It was a feat of engineering that most people in Belfast never knew about and still don’t — they simply turned on their taps and the water came through. The men who made it possible went back to their lives in the towns and villages of the Mournes.

The tunnel itself is sealed and inaccessible. You will never walk through it. But you can walk above it — and that walk is something else entirely. Beann Each (Slieve Binnian) is the third-highest mountain in the Mournes at 747 metres, and many walkers consider it the most dramatic. The summit ridge is crowned with extraordinary granite tors — jagged, otherworldly rock formations that locals once called the “Back Castles,” and folklore attributed to the Tuatha Dé Danann as otherworldly watchtowers.

The standard route starts from Carraig Beag (Carrick Little — “the small rock”) car park near Annalong, follows the Mourne Wall upward, and traverses the summit ridge past the North and South Tors. On the descent, you pass Blue Lough — a mountain lake that shifts from black to vivid blue depending on the light, tucked beneath Binnian’s eastern cliffs. The whole circuit is about 7 miles and takes 4–5 hours.

As you walk, know this: hundreds of feet beneath your boots, inside the granite, there is a tunnel that 150 men spent four years building in the dark. The water still flows through it. Belfast still drinks from it. And the mountain gives absolutely no sign that it was ever hollowed out at all.