Ran across this article in LDS Living at MA&DB.
“For generations there’s been some mention, even rumors, of a very old, secret tunnel somewhere near the temple in Nauvoo. It’s been described as having red Nauvoo brick arches and walls that are lined with carved temple stones. Some versions of the story even state that there is a sunstone on one end looking down the length of the tunnel. During the reconstruction of the temple the tunnel was not discovered and the story seemed to fade as perhaps just a legend.
Last year, during the construction of the Zions Mercantile building (right across the street from the temple) again some interest was expressed as to whether a tunnel would be uncovered as they dug their basement. The building was completed last fall, and no such find was forthcoming. That is, until a single sprinkler head needed adjusting.
When the building was all but done, landscapers were installing the final sprinkler heads. One work had some trouble getting one of the heads low enough to avoid mowers. When he tried to dig the hole a little deeper, he hit a block of white stone with carvings on two sides. After some struggle, the stone was pulled out. Removal of the stone revealed an opening. The call came to Salt Lake City, “There seems to be some kind of tunnel. All we can see are brick arches. . . but it’s very deep.”
The stone was replaced and the tunnel covered up, but not before several photos were taken of the inside. It appears to be eight feet deep and about six feet wide. It
disappears in both directions so there is no word on how long it is. Photos reveal that there are red brick arches supporting the ceiling and the walls are actually lined with the same carved stone used for the old temple. Researchers have plans to open the tunnel this summer. Several enlarged prints of the photos that were taken are on display inside the Zions Mercantile. Once the research is complete, there will be a full feature article in a future issue of LDS Living Magazine. “