The Argo Tunnel was started from its southern terminus at Idaho Springs in September 1893, and reached its final length of 4.16 miles (6.69 km) in November 1910, after several pauses in the work.

[3] The tunnel was built on an incline of one-half percent, so that water from the intersected mine workings would drain out the entrance at Idaho Springs rather than needing to be pumped out.

In addition, loaded ore cars from the mines could benefit from gravity as they were trammed downhill to the tunnel entrance and then returned uphill as empties.

It was hoped that the tunnel would intersect previously undiscovered gold veins, but there is no record that important new ore bodies were discovered.

A fifth person near the portal suspected trouble when the lights suddenly failed and ran for the entrance, narrowly escaping.