The two inner diversion tunnels have two concrete plugs in them.

Now it is possible to go all the way up the diagonal and look at the concrete plug from the underside.

It is tethered to reinforced concrete plugs anchored in the structure of the Monument.

With the exposure over time to the acidic waters that flow through the mine, this concrete plug will eventually fail after about 60-70 years of service.

Then he climbed up, squeezing under the concrete plug.

Rand smeared the concrete plug with epoxy and lowered it into place.

Of the two, Zach is the athlete, a tennis and hockey player built like a concrete plug.

Government officials said this was unlikely, pointing out that the 2,700-foot shaft had been filled with gravel and an enormous concrete plug.

The dry concrete plug was not introduced to the design until 1926.

The concrete plug is driven into the ground by the drop hammer.