The legendary innermost planet, Vulcan does not exist, but stories about it do.
Harlei: Is that why you've put it in brackets in the site navigation bar?
Zendexor: Yes - you see, I want to eat my cake and have it.
That's to say, I want Vulcan, the infra-Mercurian planet, to be on our OSS website, but at the same time I wish not to detract from Mercury's status as the innermost planet.
Hence the brackets. In effect, what those brackets are saying is: "Go ahead and think of Vulcan as the closest planet to the Sun while you're looking at it. But when you turn your attention to Mercury, shift to thinking that that's the closest to the Sun, because there's nothing between it and the Sun - except something in brackets, and that doesn't really count."
Stid: Is all this your idea of rigorous logic?
Zendexor: No, it's flexi-logic, or hedging one's bets. To continue:
Because of its closeness to the Sun, the surface of Vulcan is not regarded as habitable by those few authors who have set stories there.
So explorers have to have their adventures in caverns (as in Child of the Sun) or in the planet's spherical hollow interior (as in Outlaw World).
There seems to be no novel set primarily on or in Vulcan, though Outlaw World has its climax there.
The Vulcan of Child of the Sun is without organic life, though it is the haunt of another kind of life. The Vulcan of Outlaw World, on the other hand, has a fertile interior, a Pellucidar-type hollow, lit by a solar beam that shines through a crater opening, as Captain Future and his crew discover when he steers his ship, the Comet, down through that opening:
The whole concave inner surface was blanketed by dense, pale-green jungles. At one point glimmered the waters of a large yellow lake, into which ran several small, glittering rivers. Wispy clouds floating in the inner vault, and the whistle of air outside the falling Comet, told them that this hollow world had an atmosphere...
...The little ship whistled down toward the dense green jungle. Curt found a small opening and landed there.
The air-test dials showed that the atmosphere outside was hot, but breathable. Carrying their atom-pistols, they emerged...
They were standing on pale grass amid tall, grotesque ferns that towered high above their heads. Spiky shrubs bearing great golden flowers grew in the greenish gloom beneath the ferns. Big, bright-winged insects buzzed past. The oppressively hot air had a rank jungle smell.
Soon they meet the natives:
With a chorus of fierce yells, scores of men poured out of the jungle all around them.... These men were pale-skinned and had pure white hair. They wore white leather tunics, and carried spears. White savages, natives of Vulcan's inner world!