Until the past 50 years, Mars was generally regarded as the planet in the solar system most likely to host extraterrestrial life. This speculation gained traction in the 19th century, when astronomers began to see what they thought were canals on Mars—later revealed to be geologic formations but widely regarded, at the time, as evidence of life. Percival Lowell’s breathtaking Mars and Its Canals (1905) hypothesized that they were relics of a long-dead civilization, lost forever to time.

Percival Lowell’s 1898 illustration of the Martian “canals,” as seen by telescope.

Today, life on Mars doesn’t seem terribly likely. We’ve explored the planet for decades, and we know enough to know that most of what we send to the planet from Earth, unless encased in a greenhouse or other controlled environment, will die. But scientists do hold out hope that life could exist underneath the Martian surface, and they’ve found that hope in an Arkansas laboratory: microbiologist Rebecca Mikol subjected methanogens to conditions identical to those found on the Martian surface, and they turned out just fine.

Hope of methanogenic Martian life forms dwindled last year after the NASA Curiosity Rover found no evidence of methane on Mars, contradicting 2004 data indicating a significant amount of the stuff, but it’s entirely possible that an underground ecosystem still exists on the planet. So while the chances of current life on Mars are slim, they are greater than zero—and as more probes visit the planet’s surface, we will develop better answers to this very old question.