Since 1795, treasure hunters have poured fortunes — and six lives — into one hole in the ground off Nova Scotia. Nothing conclusive has ever come out of it.

Hundreds of giant figures were etched into a Peruvian desert two thousand years ago. We know exactly how they were made. We still don't know why.

Half of Minnesota's Brule River plunges into a hole in the rock and vanishes. For a century, everything thrown in disappeared without a trace — until two hydrologists tried the simplest test of all. One question, though, was never fully answered.

Millions of barren circles freckle the Namib grasslands in an eerily regular pattern. After decades of study, scientists still cannot agree on what makes them.

Deep in the Siberian taiga, a geologist stumbled on a 40-meter mound of shattered limestone that seemed to have burst up from below. Seventy-five years later, science still cannot say for certain what made it.

For decades, boulders crossed a dry lakebed on their own, leaving long trails and no witnesses. The answer, when it finally came, was thinner than a windowpane.

Laboratories can now name the exact woods and the distant Scottish basin the stones came from. Why they were dragged across Britain — and how — remains open.