History says that the Taj Mahal was built by Mughal emperor Shah Jahan in memory of his third wife, Mumtaz Mahal. However, I've also read many stories claiming that it was a Lord Shiva temple earlier.
What's the truth?
There are no scholarly literature (like publications in reputed academic peer-reviewed papers etc) supporting the theory that it was a temple earlier. Till such materials are available, it would be wise to consider such stories as myths.