I blame Indiana Jones. Or possibly the The Goonies. If there's a plot that I'm a total sucker for, it's the one where the protagonists go off chasing a myth that -- gasp! -- turns out to be real. It's pretty clear I'm not alone, since this is a storyline that turns up again and again: the National Treasure movies, the Mummy movies, The Da Vinci Code  by Dan Brown, the Uncharted video games, the Supernatural TV show, and probably dozens more examples.
So what's the appeal? The easy answer is familiarity. All of the things in the list above rely on that for their success, with varying degrees of cynicism. I think it's no coincidence that a lot of them aren't actually very good. "Everybody loves Indiana Jones, right? Let's do that, only different!" might be enough for a diverting 90-minutes (or 300 pages), but it's probably never going to be more than that.
I don't want to suggest, though, that the myth-chasing plot can never rise above the level of nostalgic light entertainment. I'm pretty sure I don't think that's true -- I mentioned my love for the Mellified Man story arc in my review of The Dervish House  by Ian McDonald. It's a perfect example of this sort of plot. So what makes it better than the awful National Treasure movies (aside, obviously, from the quality of the writing)?
I think the first place that myth-chasing plots fall down is that they go for the obvious targets: the Ark of the Covenant, the Spear of Destiny, Shambhala. And then, of course, they add Nazis. A big part of the joy for me in reading about the Mellified Man was that I'd never heard of it before. History is absolutely full of neat little things like that, which provide excellent leaping-off points for this sort of plot. Take the Baghdad Battery, or the sixteenth-century mechanical monk. The best of these stories seem almost unbelievable. I love being surprised by history.
When they're done well, myth-chasing stories actually give you two interrelated narratives, both of which are important. The first is the myth itself: where did it start, how did it weave its way through history, how was it changed and distorted by that passage, how did it eventually pass out of sight. The second is the story of the myth-chasers, trying to piece together this damaged and fragmentary trail.
It seems to me that those two interconnected narratives leave you with a lot of scope for interesting structural tricks. They also demand that you write intelligent protagonists, since we're talking about puzzles that have gone unsolved for a long, long time. I'd say this is a pretty common failure point for myth-chasing plots.
I'm not sure where I come down on the need for realism. Clearly I'm not a stickler for it, otherwise my love for The Mummy wouldn't be so deep. But ambiguity is often much more interesting. It's never really discussed whether the Mellified Man actually has miraculous healing powers, because it's mostly besides the point. I suppose it depends on what sort of story you're trying to tell.
I'm particularly interested in how you might use this sort of plot in fantasy or science fiction. It's easy enough to write about the Holy Grail, but what if your setting doesn't contain familiar legends to fall back on? It's a much trickier game then, particularly in anything shorter than a novel. You need to build up the myth, and then take it apart again.
Incidentally, I wonder what the Holy Grail story might look like five hundred years from now? Does the arrival of Wikipedia mean these myths will essentially cease to evolve?
I can't actually think of any examples of myth-chasing plots in fantasy or sci-fi off the top of my head, unless you include the vast number of fantasy stories which feature prophecies. Can you think of any? Suggestions welcome in the comment thread, or via ambiguously worded postcard mailed to the usual address just before the Nazis snatch you!