In Helaman 3, it says
there were an exceedingly great many who departed out of the land of Zarahemla, and went forth unto the land northward to inherit the land.
And they did travel to an exceedingly great distance, insomuch that they came to large bodies of water and many rivers.Helaman 3:3-4
It then goes on to describe all about what they did there. But how would they know? Well, that’s not the first time people went to the land northward.
And the first ship did also return, and many more people did enter into it; and they also took much provisions, and set out again to the land northward.Alma 63:7
Maybe they told the Nephites what was there, and so the Nephites guessed what the people would do. Or maybe they had a plan when they left, and that’s what they wrote down.
And they have been handed down from one generation to another by the Nephites, even until they have fallen into transgression and have been murdered, plundered, and hunted, and driven forth, and slain, and scattered upon the face of the earth, and mixed with the Lamanites until they are no more called the Nephites, becoming wicked, and wild, and ferocious, yea, even becoming Lamanites.Helaman 3:16
But why would they plan that? Of course, they wouldn’t. So throw away that theory. They still could have guessed, but I don’t think they would have. The most probable explanation is this:
There are two important things to note. First is this verse:
And now there are many records kept of the proceedings of this people, by many of this people, which are particular and very large, concerning them.Helaman 3:13
The next thing to remember is that we aren’t reading Helaman’s (the son of Helaman) words. Or Nephi’s (the son of Helaman). Or Moronihah’s (Moroni’s son). Or Hagoth’s (the guy who built the boats). Or–well, you get the point. The person writing all this down wasn’t even alive while it happened: Mormon. Mormon was abbreviating all sorts of different records. Maybe he abbreviated the people of the Land Northwards’ records too! But the Brief Explanation says
Sources from which this record was compiled include the following:
The Plates of Nephi…
The Plates of Mormon…
The Plates of Ether…
The Plates of Brass…Brief Explanation about the Book of Mormon
It never mentions the records from the Land Northwards! At first reading this, I was stumped. But then I realized it says “include”. It doesn’t ever say those 4 are the only records used. Maybe Mormon didn’t want to list them all for whatever reason. Maybe he forgot some, or maybe there were too many records, and most of them were only used for a few verses, like the ones from the Land Northward! Wait, I re-read it and there’s some information from our time that Mormon could have no way of knowing about. So that was written by the First Presidency at some point or other, and they had no way of knowing every record. So the records from the Land Northward could have been used in Helaman 3!
But how would Mormon get the record? Well, he was driven out of his land to the Land Northward.
And the Lamanites did give unto us the land northward, yea, even to the narrow passage which led into the land southward. And we did give unto the Lamanites all the land southward.Mormon 2:29
It doesn’t mention anybody else there, so, sadly, they must have been destroyed. Maybe, before they were destroyed, they put their records in the Hill Shim or the Hill Cumorah, and Ammaron and Mormon didn’t find them until it was time for Mormon to find them. He didn’t mention it though! Maybe he thought they were more records from Ammaron. Maybe he forgot to mention them. Maybe it would take too much room on the plates to mention them. We don’t know.