For nearly a century, the long meandering tracks etched into Death Valley’s Racetrack Playa remained a mystery. Clearly, some force was pushing the heavy rocks there and leaving behind these grooves. But with the remoteness of the location, it took investigators years to catch the rocks in action and solve the puzzle. For those who haven’t watched the video yet, I’ll refrain from revealing the answer here (though you can find it in previous FYFD entries)! I’ll just say that it requires all the right conditions to come together. (Image and video credit: Physics Girl; for related research see here)