Fremont Island

Pic of the island sourced from Wikipedia.

It is known by many names: mo'ko-mom-bĭtc (in Gosiute Shoshoni), Castle Island, Coffin Island, Disappointment Island, and Miller Island, but the name that has stuck is Fremont Island. It's also home to a local unsolved mystery. 

The land earned it's first English moniker of Disappointment Island in 1843 during exploration by the famous Kit Carson and John C. Fremont. This was due to it's hellish and unforgiving landscape with little vegetation and no significant wildlife to speak of. During their visit, they carved a cross into a rock which can still be seen today.

However, the mystery begins with a man named Jean Baptiste. He was arrested in 1862 for the crime of having robbed over 300 graves, being caught re-handed with robbed items in his home in Salt Lake City. After burying the stolen items in a mass grave in the city, Baptiste was brought to trial, but little to no records exist of any court proceedings. 

Supposedly, Brigham Young said that he felt hanging or shooting Baptiste would be too easy of a punishment, and life in prison "would do nobody any good", so he then sentenced him to banishment on the island.

At the time, there was a number of cattle on the island that would be attended to by occasional visits by ranchers and there was a small shack with supplies for Baptiste to live on. The plan would be for the ranchers to resupply him and check on him during their visits. He was transported there by boat, and the ranchers had subsequently visited once to find him there living off the food in the shack. 

However, after only a period of three weeks, the cattle herders visited the island only to find no sign of Baptiste. After searching the island, they found pieces of wood torn from the shack along with a cow that had been killed and tanned for leather. They postulated that Baptiste had attempted to build a raft to escape, but had never found any evidence of either his success or failure to escape, and no other official record of him exists after that time.

Big thanks to the Dead History blog for a nice write up on the topic, and they have much more detail on it, so check it out.