The Lord of the Rings trilogy is a testament to world-building and minute specificity. There is one mystery, however, that has never been fully resolved and remains perhaps the biggest unexplained question in Tolkien's universe: what happened to the entwives?
In a letter published in 1954, Tolkien wrote that he was not sure about the fate of the entwives. He guessed that they wandered eastward from Fangorn, toward Anduin and Mordor, but that their gardens would be destroyed in the Second Age by Sauron's scorched earth policy as he fought against the allies down the Anduin. This would explain why we never see or hear of them again and is perhaps the most plausible explanation of their disappearance.
Fans of the series have long latched onto this question, and there are numerous theories as to their whereabouts. The most popular is that the Entwives became huorns and live in the Old Forest, which many fans believe is actually a forest filled with huorns tended by Ents. Some people also think that Hobbits are descendants of the entwives, although that seems a little bit far-fetched.
Another theory is that the entwives remained near Mirkwood, where they would be close to their homes. But this seems unlikely, since the entwives were not huorns, and Mirkwood is a dark place, where spirits are often cast in a negative light. In addition, there are no indications that the entwives ever went this route. It is far more likely that the entwives were destroyed along with their gardens in the War of the Last Alliance and simply did not return.