How do you solve a problem like maria?

EARTH'S moon has a split personality. One half of its surface—the half which, thanks to the vagaries of orbital mechanics, always faces Earth—is dominated by dark, smooth expanses of ancient, frozen lava known as maria (early astronomers, thinking they might be bodies of water, named them after the Latin word for “sea”). The contrast between the darkness of the maria and the brightness of the surrounding highlands forms a pattern popularly known, depending on the culture of the observer, as the man in the moon, the rabbit on the moon or one of many other optical illusions.

When astronomers got their first glimpses of the moon's far side, however, they saw a strikingly different landscape. Early lunar probes revealed a surface that was mountainous, rugged, heavily cratered and virtually devoid of maria. To quote Bill Anders, one of the astronauts on Apollo 8 and thus one of the first three people to see the far side of the moon directly, it “looks like a sand pile my kids have been playing in...all beat up, no definition, just a lot of bumps and holes.”