Manapouri The Toughest Tunnel (DVD) - NHNZ and TVNZ

New Zealand’s remote Doubtful Sound,  in the south-west of the South Island, is a stunning area of tranquil  lakes, plunging fjords and dense rainforests fed by 200 inches of  rain a year.

In the 1960s a team of more than a  thousand men undertook New Zealand’s biggest ever construction  project – building a hydroelectric power station to take advantage  of the massive water supply in the region.

The men drilled and blasted a six-mile  tunnel and cathedral-sized power hall in extreme conditions – with  the cost of the lives of sixteen workers. Almost as difficult was the  twelve miles of road building needed to link the top of the tunnel  with its outlet at sea level.

Thirty years later it was decided that  a second tunnel was needed. This time, the main work was to be done  by a state-of-the-art tunnel boring machine. But the geology  challenged even this equipment. In most places the rock was so hard  that cutters had to be replaced every six feet. To make matters  worse, there were five earthquake fault bands where the hazards of  crumbling rock and constantly falling water brought the machine to a  halt and destroyed morale.

In order to link the tunnels more  drill-and-blast work was needed and bulkheads were assembled under  water. Tension reached fever pitch as the boring machine approached  the hand-built tunnel. Would the two join up?

This absorbing programme from Natural  History New Zealand covers both stages of tunnelling, with archive  footage from the 1960s and new filming from the 1990s. It shows the  skills of tunnel engineering and project management at their highest  levels