The rock carved out a circular chasm some 710 miles across. For the sake of comparison, the asteroid that careened into Earth 66 million years ago and heralded

The rock carved out a circular chasm some 710 miles across. For the sake of comparison, the asteroid that careened into Earth 66 million years ago and heralded the end of the age of the dinosaurs excavated a pit a mere 110 miles in diameter. Around 250 million years later, lava poured into this great depression, forming a sea of molten rock named Mare Imbrium, the "Sea of Rains." And as lava lapped at the shores of this magnificent desolation, a curious rock sat nearby: a mangled amalgam of crushed sediment and once-molten matter, a product of the ferocity of that ancient, colossal impact. For what seemed like an eternity, that rock sat silently on the moon, hit by the occasional micrometeorite. And 3.9 billion years after it was formed, in the twilight days of 1972, two visitors from another world, Apollo 17 astronauts Eugene Cernan and Harrison Schmitt -- Schmitt was the first and only geologist sent to the moon -- saw it glinting in starlight.