Mars has spent decades as the most plausible address for life beyond Earth, and this episode of The Universe lays out the physical evidence scientists point to. Dry channels and lakebeds carved into the surface suggest water once ran across the planet in volumes it no longer has. Ice sits frozen at both poles, confirmed by orbiter and lander instruments rather than inferred from a distance. The most debated clue is methane: its concentration in the thin Martian atmosphere rises and falls with the seasons, and on Earth methane this variable is almost always tied to living organisms, which is why some researchers argue it could signal bacteria surviving underground, insulated from the surface's radiation and cold. The film moves through these findings one at a time, letting planetary scientists explain what each does and does not prove, and treats the case for microbial life as open rather than settled. It is a status report on the search, not a verdict.