Heartlanders quote D&C 125 to show that, in their opinion, the Lord revealed the ancient location of Zarahemla:
Let them build up a city unto my name upon the land opposite the city of Nauvoo, and let the name of Zarahemla be named upon it. (D&C 125:3)
Even though there is nothing about this passage that would lead one to conclude that the Lord is identifying the ancient location of Zarahemla, in their desperation Heartlanders somehow seem to think that that is exactly the case. The more likely interpretation (and the correct one, as we will see) is that the Lord was simply naming the location after the Book of Mormon Zarahemla, much like other cities were named after ancient cities of renown (Carthage, Ill., Bountiful Utah, and Rome, TX come to mind).
My friend Tyler Livingston pointed me to a crushing argument against the Heartland interpretation of this passage, an argument by Matt Roper which I had previously overlooked. The area across from Nauvoo, which the Lord would dub “Zarahemla”, was in fact already called “Zarahemla” by some of the saints prior to the revelation of D&C 125, in March 1841. The Lord was merely using a name for the area which had already come into use by the saints, no doubt naming it after the Book of Mormon Zarahemla.
On July 2, 1839 (at least a year and eight months prior to D&C 125), Brigham Young recorded in his diary:
“Brothers Joseph, Hyrum and others came over the river to Montrose, and went out on the prairie and looked out the sight for a city for the Saints, which was called Zarahemla.”
The Prophet’s cousin, Elias Smith, also referred to a “conference at Zarahemla” in August of 1840 (that’s eight months prior to D&C 125). Matt Roper’s original publication and citations for these can be found here. The obviously correct interpretation is that the Lord was simply using the name already in use by the saints for the area, not revealing the ancient location of the Book of Mormon Zarehemla.
For more information on the community of Zarahemla, across the river from Nauvoo, see this short description at the Joseph Smith Papers: http://josephsmithpapers.org/place/zarahemla-iowa-territory
Also see here, in the Church History study guide: https://www.lds.org/manual/doctrine-and-covenants-and-church-history-student-study-guide/the-church-in-nauvoo-illinois/doctrine-and-covenants-125-zarahemla-iowa?lang=eng
And finally, here is a video produced by FairMormon which discusses this issue: