ALH 84001 is the only known specimen of a rock that was on Mars during the era when liquid water most likely existed on the planet's surface.

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ALH 84001 is a meteorite that is believed to have originated from Mars and is estimated to be about 4.5 billion years old. It contains features that suggest it formed in the presence of liquid water, indicating that Mars may have once had conditions suitable for life.

ALH 84001 Is the Only Known Specimen of a Rock