In addition to the various other roles that Missouri has in LDS history and teachings (see What is the LDS meaning of the New Jerusalem?), it is believed by most to be the location of the Garden of Eden.
The LDS basis for this belief is founded largely on the teachings of Joseph Smith in the context of Doctrine & Covenants section 57 which reads that Jackson County is to be the location of Zion and the New Jerusalem. The section does not explicitly state Jackson County is the location of the Garden of Eden, however statements by Joseph Smith, Brigham Young and others have led to this belief. Due to the unofficial nature of the teaching, not all who follow LDS teachings accept it as absolute truth, for it can be claimed that it was merely an opinion.
To many, the idea that the Garden of Eden is located in Missouri is unbiblical as it does not seem to fit the geographical description of the locale as given in Genesis. John A Widtsoe, 20th Century LDS Apostle and scholar, addressed this issue in his book, "Evidences and Reconciliations" ("http://archive.org/details/evidencesreconci03widt"):
Despite the apparently specific descriptions given, this clue has not led to the location of the Garden of Eden. Careful scholars have not been able to identify any of the four rivers with certainty. None of the rivers mentioned fits into the lands now known. Since the historically well known names of Euphrates, Assyria, and Ethiopia do not fit into the use of them in the Garden of Eden story, it is more than probable that they are ancient names variously applied in later times. Clearly, these rivers and countries belong to early ages of the world's history, and do not apply to present-day terminology.
Although there is no perfect match for the geography of the Garden of Eden in Missouri, it is my personal opinion that of the geographical locations in the United States, Missouri perhaps is one which would best fit the discriptions. This Map from wikipedia demonstrates the large number of rivers in and around Missouri in general. As Widtsoe mentioned about the earth in as a whole, It is very probable that this area looked much different in the Antediluvian era; variations in river flows could easily account for the geographical descrepancy.