Conditions must be perfect. The right amount of mist, cloud, sun and a climber walking the slopes of a mountain will create an optical illusion called the Spectre of the Brocken.
It's an elongated reflection with a rainbow halo, sometimes called a glory, around the climber's head.
It is so named after the Brocken - the highest peak of the Harz mountains in Northern Germany, where the spectre was first recorded.
Legend has it that a climber was so frightened by the sudden appearance of a human figure with a ring of light around its head he fell to his death.
Stratford's Baz Preston had not heard of the spectre either when he first saw it in 1949, at Windy Point on Nelson's Dun Mountain.
"I thought it was pretty spooky."
Later, when he became a collector of mountaineering books, he read all about it.
Mr Preston, who has climbed Mt Taranaki more than 600 times, says the spectre is similar to a rainbow. Three things have to line up for it to happen - the sun, the person and cloud or mist in the background.
"It is your shadow that appears on the cloud and the big rainbow halo forms around your shadow. You can see your reflection. It depends on the density of the cloud on how brilliant the spectre's going to be."
He has often seen spectres on mountains in the North Island, but not so much in the South Island."
Mr Preston spent four seasons climbing in the Southern Alps and did not see it once. And he knew the Spencer Mountains in Nelson like the back of his hand, but he never saw it there either.
But he doesn't agree that sightings are rare on Mt Taranaki, having seen the spectre about 27 times.
"There was one time I was coming down the mountain, I saw one on the summit and all the conditions were right, the sun was breaking out of the cloud and that happened all the way down the mountain. It appeared about a dozen times."
On another trip he was coming down Mt Taranaki on a day when the mountain was covered in cloud and visibility was quite low - maybe about 10 metres at most.
"I caught up with some Germans. I think from memory there were about 10 of them and I could see they were having trouble, so I asked if they wanted to come down with me. I told them I come up here quite a bit."
They were walking down the track, with Mr Preston in front, when they came around a bend and the sun broke through the cloud, he says.
"I could see the mist welling up and I said, "Get out your cameras, we are going to see a spectre ahead."
They asked him how he knew, and managing to keep a straight face, he told them the spectre followed him around the mountain.
The spectre turned out to be the best one he had ever seen. After they had taken photos the tourists chatted among themselves in German for a minute before one of them came over to Mr Preston.
He put his hand on Mr Preston's shoulder and told him seriously that they thought they would probably have seen the Brocken Spectre even if he had not been with them.
He believes it appears on Mt Taranaki because the mountain is a cone, the bush is quite low down in comparison with the height of the mountain, and it's surrounded by sea, meaning the mountain draws a lot of cloud.
After years of hearing Mr Preston talk about the spectre, his wife, Selma, finally got to see one herself on a walk up past the Stratford Plateau car park.
When they arrived it was very foggy, but Mr Preston estimated it would clear further up. About five minutes up the road they broke through the mist and their shadows went across the road.
"I guaranteed we'd see the Spectre of the Brocken and we did."
Mr Preston has been a "bit of a mountain freak" since he was a teen.
"My dad was a keen hunter and I went out hunting with him and I got to the stage where I was more interested in the country and the mountains than shooting game. I guess I was about 14 or 15 then."
He's going on 79, now.
His first mountain was Wooded Peak near Dun Mountain in Nelson.
When he came to Taranaki in 1975, he was amazed at how everyone kept track of how many times they had climbed the mountain.
"Everyone kept a record of the number of ascents. I never bothered about that. At a conservative estimate I've climbed it over 600 times.
"It's part of my psyche, I suppose. It's always different, it's never the same. The weather is always changing. I regard it as the weather- maker."
"Once I knew the conditions were right for the spectre to appear, I would go out to the top of the ridge so my shadow would appear on the cloud. There is always cloud coming up. Especially Hongi's Valley on the mountain. It sort of creates the optimum conditions for a spectre."
He doesn't know why he is so entranced by it, he says. "It's kind of a spiritual sort of thing."
Once on the Ruahines, one of his sons saw the spectre with him. "And he would have only been about 9 at the time, but he still remembers it."
Mr Preston has lots of memories, photos and slides of the spectre. He is hoping to talk his two sons into taking him for one last climb on the mountain - one last chance to see the spectre.
Climbing on his own came to end four years ago when he had a bad fall. Turning 75, he thought a good challenge would be to climb the mountain 75 times in the one year.
"I nearly got to August and I realised I'd have to do winter ascents. So I decided to climb the smaller peak five times just to get confidence. I was getting on in years so I had eased off winter climbing. I'd done it four times so I had to do it one more time. I decided to walk around the mountain from the Stratford Plateau instead of going to Dawson falls. I figured that would take six or seven hours, similar to a summit climb in the winter."
But he slipped on some icy steps and had a bad fall, hitting the back of his neck, shoulders and lower back. He managed to walk out but was in bad shape for a week.
Still keen to do his 75 climbs, Mr Preston took up race walking to keep fit, a sport he had done previously.
He placed third in the Taranaki Daily News half-marathon then got "carried away" walking three marathons and numerous half-marathons. He overdid it and ended up in hospital for three months.
His wife nursed him back to health, he says, but now he is a bit nervous about going up the mountain.
"I haven't climbed a mountain for 3 years."
- Taranaki Daily News
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