Aragorn's first visit to Moria

[The Mouth of Sauron]

As the Company approached Moria, Aragorn stated that he had been in Moria once before.

Any ideas on why he might have gone there before ?

[Galadriel55]

Maybe he was accompanying Gandalf when he went there?

[Alcuin]

Aragorn had entered Moria from the east, through the Great Gates in Dimrill Dale. When he did this, we are not told. I have read some speculation that he might have lost a companion, such as another Ranger, in Moria. In any event, when the Company of the Ring fled after Gandalf fell into the abyss, there was a company of orcs guarding the entrance. We do not know if this was typical after the orcs re-established themselves following the Battle of Azanulbizar: perhaps they were watching for travelers through the Mines (i.e., the Company of the Ring), or perhaps it was normal procedure. Somehow, Aragorn got in, alone or (more likely) with others; but he escaped only with difficulty, and the memory of the experience was so black, he was willing to attempt crossing the Redhorn Pass in the dead of winter knowing that his transit was probably being watched.

Gandalf said that only he had passed completely through the Mines. Since he exited through the Doors of Durin, he must also have entered from Dimrill Dale. And since he alone had transited the Mines, Aragorn cannot have accompanied him, giving us the additional information that Aragorn also entered and left through Dimrill Dale.

I cannot recall reading in Silmarillion, Unfinished Tales, Letters, History of Middle-earth, or Reader’s Companion of any further exposition of Aragorn’s (mis)adventures in Moria. If anyone is aware of anything else Tolkien wrote about this, please post it!

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added: After Boromir carped about opening the Doors of Durin to enter Moria, Gandalf told him he had entered from the east.

[Inziladun]

Originally Posted by Alcuin

In any event, when the Company of the Ring fled after Gandalf fell into the abyss, there was a company of orcs guarding the entrance. We do not know if this was typical after the orcs re-established themselves following the Battle of Azanulbizar: perhaps they were watching for travelers through the Mines (i.e., the Company of the Ring), or perhaps it was normal procedure.

I've always connected the strong Orc-guard on the east-gate to the Company's presence in Moria. I can see them certainly keeping a watch on the Dimrill Dale under normal circumstances, but that's probably all that was required. As long as they had a means to quickly assemble all their forces in Moria if need arose, I don't see why they'd have needed to retain such a strong guard constantly.

Originally Posted by Alcuin

Gandalf said that only he had passed completely through the Mines. Since he exited through the Doors of Durin, he must also have entered from Dimrill Dale. And since he alone had transited the Mines, Aragorn cannot have accompanied him, giving us the additional information that Aragorn also entered and left through Dimrill Dale.

I too, doubt that Aragorn and Gandalf made their journey into Moria together. The dialog between the two doesn't seem to support that. Aragorn almost certainly did enter from the east like Gandalf, though, as neither knew the password to open the west-gate.
However, where does it say that only Gandalf had made the full journey through Moria? Aragorn says of his encounter there:

'I too once passed the Dimrill Gate, but though I also came out again, the memory is very evil.'

I don't think one can extrapolate from that that Aragorn didn't make it all the way thorough.

As for the original question of why Aragorn entered Moria, ask yourself, 'Why did Gandalf"? Lacking an exploratory motive, both would have merely been looking for a shortcut through the Mountains at some point, for whatever reason of needing haste.

[Puddleglum]

Originally Posted by Inziladun

I don't see why they'd have needed to retain such a strong guard constantly.

As for the original question of why Aragorn entered Moria, ask yourself, 'Why did Gandalf"?

Just a few quick thoughts.

• Keeping a guard seems to be a very normal thing in a dangerous world. The Elves of Lorien did it; The Dwarves of Erebor did it; The Bree-landers did it; The Bucklanders did it (even before the Ruffians took over); The Rohirrim did it; The men of Minas Tirith did it; The orcs of Cirith Ungol did it (until distracted by in-fighting); Minas Morgul did it; Saruman did it in Isengard; and so on. While Saruman's & Sauron's search for the Ring may have (I suspect "DID") play a part - the presence of a guard seems very plausible to me in any event.
• Gandalf spent a fair amount of time search high & low for Thrain. That included even venturing into Dol Guldur and the dungeons of the Necromancer. Given the connection between Thrain and Moria (he wanted to venture in after the battle of Nanduhirion), Gandalf could well have explored there himself as part of his search.
• Perhaps Aragorn ventured in as part of his hunt for Gollum. He doesn't tell much about his search after Gandalf gave up (other than tracking him to the confines of Mordor and finding him in the Dead Marshes), but it is at least possible he searched in Moria also.

[Inziladun]

Originally Posted by Puddleglum

Keeping a guard seems to be a very normal thing in a dangerous world. The Elves of Lorien did it; The Dwarves of Erebor did it; The Bree-landers did it; The Bucklanders did it (even before the Ruffians took over); The Rohirrim did it; The men of Minas Tirith did it; The orcs of Cirith Ungol did it (until distracted by in-fighting); Minas Morgul did it; Saruman did it in Isengard; and so on. While Saruman's & Sauron's search for the Ring may have (I suspect "DID") play a part - the presence of a guard seems very plausible to me in any event.

The Orcs apparently weren't that alert for intruders, at least not from both directions, east and west. There is no indication they were aware enemies were in the Mines until Pippin dropped the stone into the well. As I said, I can see the purpose of keeping a watch, but not necessarily a strong force like the one the Company encountered.

Originally Posted by Puddleglum

Gandalf spent a fair amount of time search high & low for Thrain. That included even venturing into Dol Guldur and the dungeons of the Necromancer. Given the connection between Thrain and Moria (he wanted to venture in after the battle of Nanduhirion), Gandalf could well have explored there himself as part of his search.

Ah, you're right there. Gandalf indeed told the Fellowship that's why he had gone to Moria.

Originally Posted by Puddleglum

Perhaps Aragorn ventured in as part of his hunt for Gollum. He doesn't tell much about his search after Gandalf gave up (other than tracking him to the confines of Mordor and finding him in the Dead Marshes), but it is at least possible he searched in Moria also.

The trouble with that idea is that Aragorn's search for Gollum apparently didn't begin until Gandalf asked him to help TA 3001 (The Tale of Years). Balin's Dwarf-colony had been destroyed in 2994, and the Orcs had obviously returned to Moria at that point.
When the Company was debating whether to go through Moria, Gandalf says this:

If there are Orcs there, it may prove ill for us, that is true. But most of the Orcs of the Misty Mountains were scattered in the Battle of Five Armies.

If Aragorn had been through Moria recently enough to have looked there for Gollum, shouldn't he have known there were Orcs there, and warned the Fellowship?