1 GM: By the way, Dwalin, you detect that the passage is sloping.
2 Dwalin: {in pitch blackness} Uip or doon? {translation: Up or down?}
3 GM: It just says you have a 75% chance of detecting sloping passages. Nothing about up or down.
4 Dwalin: Well fat lo' o' guid tha' is! {translation: Well fat lot of good that is!}
4 Mordekai: Which way should we be going: up or down?
4 Dwalin: I dinnae ken! {translation: I don't know!}

I tried to find the relevant rule for this in Dungeons & Dragons 3rd Edition, but it reads:

Stonecunning: This ability grants a dwarf a +2 racial bonus on Search checks to notice unusual stonework, such as sliding walls, stonework traps, new construction (even when built to match the old), unsafe stone surfaces, shaky stone ceilings, and the like. Something that isn't stone but that is disguised as stone also counts as unusual stonework. A dwarf who merely comes within 10 feet of unusual stonework can make a Search check as if he were actively searching, and a dwarf can use the Search skill to find stonework traps as a rogue can. A dwarf can also intuit depth, sensing his approximate depth underground as naturally as a human can sense which way is up. Dwarves have a sixth sense about stonework, an innate ability that they get plenty of opportunity to practice and hone in their underground homes.*

Good lord! How insanely overpowered is this ability?! A dwarf can automatically tell how far underground he is at any time, as easily as a human can tell which direction is up? Can you say, "This ability completely hoses any respectable dungeon adventure design!"? Not to mention comic scripts.

So I went back to my trusty 1st Edition Advanced Dungeons & Dragons Players' Handbook, which states that dwarves merely have a 75% chance of detecting sloping passages. Phew, saved by legacy systems again.

See, as you get older, you get wiser. Meaning, you know more stuff and can put it to good use. These young whippersnappers creating gaming comics these days who never cracked a D&D book older than 2nd Edition are missing half the good jokes.

* This text is Open Game Content, and is licensed for public use under the terms of the Open Game License v1.0a.

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2017-08-09 Rerun commentary: In the latest, 5th edition of Dungeons & Dragons, the analogous dwarven ability reads:

Stonecunning: Whenever you make an Intelligence (History) check related to the origin of stonework, you are considered proficient in the History skill and add double your proficiency bonus to the check, instead of your normal proficiency bonus.

That's it. Nothing about detecting slopes or unsafe passages or stonework traps or depth underground or anything. All you get is the ability to discern the origin of stonework - like if it was created by dwarves, or goblins, or tunnelling monsters, or something else. And it's not even automatic - all you get is double proficiency bonus on the skill roll.

I'm glad that Wizards of the Coast took my advice to heart and reduced the overpowered Stonecunning ability of 3rd edition dwarves! You can thank me for that one, folks!