Is it aliens? Flowing water? Or perhaps the long-awaited discovery of Martian crabs?
NASA sent space fans into a frenzy on Sunday by announcing it will hold a special news conference to reveal "a major science finding" from the agency's exploration of Mars.
NASA's big Mars announcement
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NASA's big Mars announcement
Speculation is rife following NASA's promise that it will soon brief the world on a great 'Mars mystery' it has solved.
The space agency says it has solved a major "Mars mystery" and will reveal all during the news briefing from headquarters in Washington, DC, at 11.30am on Monday (1.30am on Tuesday in Sydney).
NASA's "special announcements" don't usually disappoint.
One such announcement in July was the discovery of a new planet, Kepler-452b, found orbiting a star very much like Earth's own sun.
NASA has asked people to submit questions for Monday's press conference via Twitter, using the hashtag #AskNASA.
Space fans have already bombarded the space agency with frenzied speculation over what the announcement might be about.
Many are hoping it may be the discovery of Martian life. Or possibly a development relating to the odd crab-like object seen in one image taken from the Curiosity rover.
Others are wondering if NASA has finally found Mark Watney, the fictional astronaut played by Matt Damon in The Martian who became stranded alone on Mars.
@NASA You sly buggers. Making us think you're going to disclose aliens then boom - it's something about a rock or whatever :P— Rudie95 (@Ruderzz) September 25, 2015
. @nasa Did you find evidence of potatoes on Mars?!?!— Sally Monster (@sally_monster) September 26, 2015
The most credible speculation seems to be that NASA will announce the discovery of flowing water on Mars.
The biggest hint is that one of five speakers at the news briefing will be Lujendra Ojha from the Georgia Institute of Technology in Atlanta.
Ojha made headlines in 2011 when he co-authored a study suggesting that liquid water flowed during the warmer months on Mars.
He said at the time that, by accident, he noticed irregular features in images taken for another study of gullies in Mars craters.
Using a computer algorithm to monitor changes over time, he began to see "finger-like" features and streaks that strongly resembled water. They would appear during warmer seasons and die away during cooler seasons.
He has conducted research ever since, to determine if it is definitely water.
Another Georgia Tech grad student, Mary Beth Wilhelm, is also tabled to speak alongside Ojha.
Other clues that liquid water may exist on Mars emerged earlier this year, when researchers believed they may have found evidence of salty water at some times of the year.