In the days of Lib the people "built a great city by the narrow neck of land, by the place where the sea divides the land" (Ether 10:20). If the term "narrow neck of land" can be equated with "the small neck of land" found in Alma 22:32, where "the small neck of land" was "between the land northward and the land southward" such that "the land of Nephi and the land of Zarahemla [which were part of the land southward] were nearly surrounded by water" (Alma 22:32); then this gives possible meaning to the words "by the place where the sea divides the land" (Ether 10:20).
The Book of Mormon geography student might take caution in the fact that this "great city" was described as being built BY the narrow neck of land and not IN it, and BY the place where the sea divides the land and not AT it. One should also take caution in the fact that the terms "the narrow neck of land" and the "small neck of land" are never specifically termed an "isthmus." Additionally, we can't be certain that the terms were ever capitalized. If they were only uncapitalized descriptive phrases, it is hard to know whether the two terms positively describe the same area. [Alan C. Miner, Personal Notes]