Lucky Starr and the Big Sun of Mercury by Isaac Asimov

perhaps go over them with us?"
    "Yes, of course."
    "Now as far as you know, Dr. Cook, the mines are in good shape, I hope. I mean, there's no danger of collapse or anything like that?"
    "Oh no, I'm sure there's nothing of the sort possible. We're built right over some of the shafts, and we had to look into the engineering when the Observatory was first being set up. The shafts are well-buttressed and completely safe, particularly in Mercury's gravity."
    "How come," asked Bigman, "the mines were shut down, if they're in such good shape?"
    "A good question," said Cook, and a small smile broke through his expression of settled melancholy. "Do you want the true explanation or the interesting one?"
    "Both," said Bigman at once.
    Cook offered smokes to the others which were refused, then lit a cigarette after tamping it on the back of one hand in an abstracted manner. "The truth is this. Mercury is quite dense, and there were hopes that it would be a rich source of the heavy metals: lead, silver, mercury, platinum. It was, too; not as rich as might be, perhaps, but rich enough. Unfortunately, it was uneconomic. Supporting the mines here and transporting the ore back to Earth or even the Moon for processing raised the price too high.
    "As for the interesting explanation, that's another thing completely. When the Observatory was first set up fifty years ago, the mines were still a going concern, though they were already closing down some of the shafts. The original astronomers heard stories from the miners and passed it on to the newcomers. It's part of the Mercurian legendry."
    "What are the stories?" asked Bigman.
    "It seems miners died in the shafts."
    "Sands of Mars!" cried Bigman testily. "They die anywhere. You think anybody lives forever?"
    "These were frozen to death."
    "So?"
    "It was a mysterious free/ing. The shafts were fairly well heated in those days, and their suit power units were in operation. The stories accumulate embroidery, you know, and, eventually miners wouldn't go into the main shafts in anything but gangs, wouldn't go into the side shafts at all, and the mines shut down."
    Lucky nodded. He said, "You'll get the plans for the mines?"
    "Right off. And replacements for that inso-suit too."
    Preparations proceeded as though for a major expedition. A new inso-suit, replacing the one that had been slashed, was brought and tested, then laid to one side. After all, it would be ordinary space-suits for the dark-side.
    The charts were brought and studied. Together with Cook, Lucky sketched out a possible route of exploration, following the main shafts.
    Lucky left Bigman to take care of packing the adjunct-units with homogenized food and with water (which could be swallowed while still in the suit), make sure of the charge of the power units and the pressure on the oxygen tanks, inspect the working efficiency of the waste disposal unit and the moisture recirculator.
    He himself made a short trip to their ship, the
Shoot
ing Starr.
He made the trip via the surface, carrying a field pack, the contents of which he did not discuss with Bigman. He returned without it but carrying two small objects that looked like thick belt buckles, slightly curved, in dull-steel finish and centered by a rectangle in glassy red.
    "What's that?" asked Bigman.
    "Microergometers," said Lucky. "Experimental. You know, like the ergometers on board ship except that those are bolted to the floor."
    "What can these things detect?"
    "Nothing at a couple of hundred thousand miles like a ship's ergometer, but it can detect atomic power sources at ten miles, maybe. Look, Bigman, you activate it here. See?"
    Lucky's thumbnail exerted pressure against a small slit in one side of the mechanism. A sliver of metal moved in, then out, and instantly the red patch on the surface glowed brightly. Lucky turned the small ergometer in this direction and that. In one particular position, the red patch glowed with the energy of a nova.
    "That," said Lucky, "is