Few plants are as mysterious to scholars as Silphium. Described as a giant fennel, this plant was native to the Cyrene area (now part of Libya) where it was apparently quite abundant. The historical record didn't have much to say about it, though, until a group of Greek colonists arrived in 630 B.C.E. and went absolutely bonkers over it. They got rich selling the plant all across the Mediterranean and developed such an affinity for it that they put it on their coins.
The question was: Why exactly was Silphium so popular? Historians have a number of theories, including that it was used as a garnish for food or as a medicine to treat common symptoms like fever and abdominal pain. But for many scholars, those don't seem like important enough uses to warrant such a craze. The real reason might have been a bit more taboo: Maybe it was used for birth control.
Whatever the case may be, we'll never know because there isn't any Silphium left to test. Sheep might be to blame for overgrazing the plants, or the Cyreneans may just have picked it all. Either way, the natural philosopher Pliny the Elder said someone found the last stalk during his lifetime and gave it to the Roman emperor Nero. If that's true, it places the extinction date somewhere around 50 C.E. [source: McCarthy].