San Francisco, March 13 : Elon Musk on Monday reacted to a post in which a user mentioned that Twitter CEO is planning to build a city on Mars, and called it a "top secret". When a user posted, "BREAKING: Elon Musk is planning to build a city on Mars. @elonmusk", Musk replied: "Top secret." Several users expressed their thoughts on Musk's post. While one user said, "Take me I hate it here", another commented, "make sure you give it a really good name when you found it". In August last year, Musk had said that he hopes for a self-sustaining city on the Red Planet in 20 years' time. Last July, the tech billionaire had said he was optimistic that "humanity will reach Mars in your lifetime". "Without a common goal, humanity will fight itself. The Moon brought us together in 1969, Mars can do that in the future," Musk had said. /IANS