It sounds like the plot of a science fiction film, but it could possibly come true: If NASA brings Martian dirt samples back to Earth, says a new report by the National Research Council, they could contain previously unseen bacteria or viruses that spread like wildfire, endangering the planet. Both NASA and the European Space Agency have discussed missions that would bring soil samples back for more detailed testing. The report warns, though, that any Martian soil must be strictly quarantined to prevent the possible spread of alien microbes. “Given that this is a very high-stakes game where we’re talking about a potentially global problem, we have to be inherently conservative,” report chairman Jack Farmer tells New Scientist. “I think the bottom line here is containment, containment, containment.” A proper isolation facility could take a decade to build, says the committee, so NASA should begin designing it now.