(uplifting music) - [Narrator] In October of 1953, work began on the outlet works, which consists of six flood control tunnels.

Fearing that dynamite explosions would disintegrate the heavily fractured shale rock, a new 85 ton continuous mining machine was developed to drill the tunnels.

This drilling innovation was called the Mitri Mole and was able to advance the tunnel's length more than two feet per hour.

The six flood control tunnels are each 18 and one quarter feet in diameter and approximately 3,200 feet long with an intake structure for each tunnel.

In April of 1957, the last of six flood control tunnels was opened.

The stealing basin for the outlet works covered 7 acres and was excavated to a depth of 200 feet.

This required removal of 3.5 million cubic yards of material.

The primary basin's concrete slab was built to a uniform thickness of 8 feet.

A similar drilling machine, the Prairie Miner was used in construction of the powerhouse tunnels.

This phase of work was completed by March of 1961.

Above the powerhouse tunnels, the core constructed a 145 foot tall intake control structure to house the control gates for each of the seven power tunnels.

By 1958, work began on the powerhouse sub-structure.

More than 222,000 cubic yards of concrete was required.

This work employed 800 men working in three shifts around the clock.

(uplifting music)