Identify short story from Year's Best Science Fiction, where school friends descend an ancient stairway into a deep ravine looking for adventure

This was a short story/novella published in Gardner Dozois's "The Year's Best Science Fiction" series. It was probably included somewhere around the 2003 20th edition, give or take a number of years.

The story is about 3 or 4 high school friends, one female, who embark on a journey of discovery by traversing down an ancient stairway into the depths of a steep deep ravine to search for the remaining traces of the "ancients." The friends don't really know where their lives are headed and are each descending the ravine in search of some inner truth.

Near the top, the stairway is well maintained but as they descend deeper it becomes more ancient and dilapidated until it is barely traversable. As they near the bottom, where the ancients had their cave dwellings, one person finds his inner strengths, one wills himself out of existence, and the female finally understands everything.

Believe me, the story is magnitudes better than my description of it.

Can you please identify it?

Answer

This is The Edge of the World by Michael Swanwick, and it's in The Year's Best Science Fiction - 7th Annual Collection edited by Gardiner Dozois. I also found a copy on the Fantasy Magazine web site.

The children are Donna and Piggy and Russ. It isn't clear exactly where the story is set, but it's in a country in the Middle East somewhere. It also isn't clear what the cliff is though (obviously) it's a cliff that apparently has no bottom.

The children descend the cliff out of boredom rather than any desire to seek the wisdom of the ancients. A long way down they find a ruined monastery dug into the cliff wall, and it's there that the weirdness starts.

Russ wishes himself out of existence:
She cleared her throat. “Russ? What do you wish?”
In the bleakest voice imaginable, Russ said, “I wish I’d never been born.”
She turned to ask him why, and he wasn’t there.

“Hey,” Donna said. “Where’d Russ go?”
Piggy looked at her oddly. “Who’s Russ?”

and:

Donna wishes for an understanding of the world
“If I could wish for anything, you know what I’d wish for?”
“Bigger tits?”
She was so weary now, so pleasantly washed out, that it was easy to ignore Piggy. “I’d wish I knew what the situation was.”

that she later regrets:

She knew exactly what the situation was.
Dear God, she prayed, let it be that I won’t have this kind of understanding when I reach the top. Or else make it so that situations won’t be so painful up there, that knowledge won’t hurt like this, that horrible secrets won’t lie under the most innocent word.