"Over the mountains Of the Moon, Down the Valley of the Shadow, Ride, boldly ride," The shade replied— "If you seek for Eldorado!"
In the poem's final stanza, the shadow responds to the knight. Oh boy, this oughta be good!
The knight learns that he must ride over the "mountains / Of the Moon" and "Down the Valley of the Shadow" if he's looking for Eldorado.
Gee, thanks a lot for the useless advice, Señor Shadow. He doesn't say where Eldorado is, only that the knight should probably ride over the mountains and down the valley.
Still—mountains of the Moon? Valley of the Shadow? What and where are these strange places?
Well, let's think about that for a second. We'll start with those mountains.
You see, a long time ago people didn't know where the source of the Nile River was. (No internet, no National Geographic, none of that stuff. Tragic, we know.)
So, ancient geographers thought the source was somewhere in a snow-capped range of mountains in central Africa, which they called the Mountains of the Moon (because of their snow). Get it? Moon=white=snow. (These mountains today are known as the Ruwenzori Mountains.)
The Valley of the Shadow is a little more abstract. It's not on any map or GPS we've heard of. For that reason, we think it's the shade's way of referring to death. This is because the phrase itself comes from the Bible: "though I walk through the valley of the shadow of death, I will fear no evil: for thou [art] with me; thy rod and thy staff they comfort me" (Psalm 23:4).
(Oh, and also this "valley of the shadow of death" thing was famously used in a song by Coolio in the movie Dangerous Minds.)
Anyway, to get back to those mountains and that valley: just what the heck are they doing here?
Well, the mountains are really far away (in Africa), somewhere that it's not easy to get to. They're almost mythical. As well, the reference to Psalm 23:4 would indicate that the only thing that can come of finding Eldorado is death.
So, it sounds like Eldorado doesn't really exist at all (which is why we think it's rhymed with "shadow" throughout the poem).
Perhaps this is the speaker's way of suggesting that, if one goes through life expecting to find something super-awesome like Eldorado, one will be disappointed. Maybe, just maybe, it's all about the journey itself.
No Eldorado? No streets of gold? Just a big ol' heaping of death? Talk about a down ending!