July 21, 1969 — Neil Armstrong steps on Moon at 2:56:15 AM (GMT)

Eldorado

Gaily bedight,
A gallant knight,
In sunshine and in shadow,
Had journeyed long,
Singing a song,
In search of Eldorado.

But he grew old—
This knight so bold—
And o'er his heart a shadow
Fell as he found
No spot of ground
That looked like Eldorado.

And, as his strength
Failed him at length,
He met a pilgrim shadow—
«Shadow,» said he,
«Where can it be—
This land of Eldorado?»

"Over the Mountains
Of the Moon,
Down the Valley of the Shadow,
Ride, boldly ride,"
The shade replied—
«If you seek for Eldorado!»

by E.A.Poe (1849)

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What's «the Mountains of the Moon»???

Your best guess is that Poe was thinking about Mountains of the Moon (Africa).

However, keep in mind that Poe was writing for Americans of the XIXth century, and they most likely didn't know much about African geography

It is possbile that the poet had something else on his mind… It is the Great Moon Hoax!

«The Great Moon Hoax» refers to a series of six articles that were published in The Sun, a New York newspaper, beginning on August 25, 1835, about the supposed discovery of life and even civilization on the Moon. The discoveries were falsely attributed to Sir John Herschel, perhaps the best-known astronomer of his time.

According to legend, The Sun's circulation increased dramatically because of the hoax and remained permanently greater than before, thereby establishing The Sun as a successful paper. It was not discovered to be a hoax for several weeks after its publication and, even then, the newspaper did not issue a retraction.

The story also have inspired Edgar Allan Poe to write and publish «The Balloon-Hoax» on April 13, 1844.

Poe had published his own Moon hoax in late June 1835, two months before the similar Locke Moon hoax, in the Southern Literary Messenger entitled «Hans Phaall – A Tale,» later republished as «The Unparalleled Adventure of One Hans Pfaall.» The story was reprinted in the New York Transcript on September 2–5, 1835, under the headline «Lunar Discoveries, Extraordinary Aerial Voyage by Baron Hans Pfaall.» The story is regarded as one of the first science fiction stories. Poe described a voyage to the Moon in a hot-air balloon, using a factually plausible scenario: Pfaall lives for five years on the Moon with lunarians and sends back a lunarian to earth. The Poe Moon hoax was less successful because of the satiric and comical tone of the account. Locke was able to upstage Poe and to steal his thunder. In 1846, Poe would write a biographical sketch of Locke as part of his series «The Literati of New York City» which appeared in Godey's Lady's Book.

Another words, Poe was NOT writing about some African mountains in «Eldarado»