Martians are primitive but intelligent humanoids who lived beneath the sands of Mars. Martians burst into flames when brought in contact with water. Martians killed many of the early human explorers on Mars, principally because they concealed their existence, and they weren't suspected.
The Martians were wiped out when Jack Brennan caused an ice asteroid to crash into the surface of Mars, raising the average humidity of the atmosphere. (As a Protector - even a Human one - Brennan viewed the number of humans already killed by Martians, and the prospect that the Martians might some day kill another, as fully sufficient grounds to eradicate their entire species.) Some Martians still exist on the "Map of Mars" on the Ringworld.