On September 10, 2011, NASA launched GRAIL—twin spacecraft nicknamed Ebb and Flow—on a mission to peer deep beneath the Moon's battered crust. Instead of optical cameras, their primary payload was an exquisitely sensitive Ka-band microwave link measuring microscopic fluctuations in the distance between them as they chased each other in low lunar orbit. When the lead probe drifted over a subterranean mass anomaly, it surged forward; the trailing probe measured the delta with micrometer precision. This high-stakes orbital ballet yielded the highest-resolution gravity map of any celestial body, radically rewriting our understanding of lunar volcanism and planetary collisions. For modern space engineers, GRAIL was a triumph of distributed sensing, proving that mapping an alien world's interior doesn't require drilling its mantle—just extraordinary telemetry and two robotic sentinels locked in synchronized orbit.