I had a team of experienced divers up top including a nominated rescue diver fully suited up. It was a private dive trip with friends. Boat was anchored offshore, we went ashore in a dinghy. We dropped a shot line down the vertical shaft and tied it off up top. I volunteered to be the guinea pig and lowered myself down the shot line to the bottom at about 10 meters. I had no fins on so as not to stir up silt.
At the bottom there was too much fallen debris to proceed out horizontally, the mine runs out under the sea floor. Visibility was great, it was midday so the sun was overhead, I remember the surreal blue-green light vividly.
I poked around for a while, considered tying the shot line to some of the junk so we could haul it out of there but decided that would turn it into a multi day project and we had other fish to fry. Went back up the shot line, total dive time was around 20 minutes.
Just another lovely summer day diving around New Zealand's offshore islands. I was a very active diver at the time, certified as a PADI divemaster circa 1988