Silphium was the legendary plant from antiquity. Its heart-shaped fruit was a cure-all that was used as a remedy for many conditions, including warts, pneumonia, typhus, fever, and indigestion, as well as an aphrodisiac.
Most famously, it was supposedly used as to prevent pregnancy and even induce abortion.
It is supposedly extinct, but there is some numismatic evidence for its previous existence. (Be sure to scroll all the way down the page to see several coins with the silphium plant on the link below:)
http://www.wildwinds.com/coins/greece/kyrenaica/t.html
A healthy skeptic would not fail to mention that there is also numismatic evidence for many mythological gods and other entities, but that doesn't prove their existence, either.
This plant purportedly only grew along a small region of the modern Libyan coastline known as Cyrenaica. The city of Cyrene was important for its trade and cultivation. Many coins from the city of Cyrene portrayed its image.
Ancient writers ranging from Herodotus, Strabo, Catullus, and Pliny mention it.
The Ancient Roman physician Soranus even recommended it's use for contraception.
So why do I doubt its existence with all of its numismatic and literary evidence for this extinct plant? (I'm committing numismatic heresy here.)
These are some of the many reasons for my doubts:
First, I'm skeptical of any medicine which reportedly has such an unlimited potential for so many unrelated conditions, ranging from the treatment of warts to its use for contraception.
Second, Ancient "global warning" has been blamed for the extinction of the plant. The argument is that Ancient Roman, similar to the modern Western World, is the source of all destruction.
Third, the argument is that the plant became extinct because it only could be cultivated in Cyrenaica and those [evil] Romans exhausted the supply of this priceless plant. Oh, isn't that nice. Supposedly, Nero (you remember, that gluttonous brute) was given the last stalk of the plant (and presumably ate it). This is a nice story, but too "convenient." So a plant as valuable as silphium couldn't grow anywhere else?
Fourth, the argument goes that between the confusion of war and excessive grazing by livestock, silphium became extinct. That is sort of like the "dog ate my homework" argument.
According to Pliny the Elder:
http://www.perseus.tufts.edu/hopper/text?doc=Plin.+Nat.+19.15&redirect=true
Fifth, it is easy and fun to believe in mythical creatures and places and even plants. Unfortunately, reality can be harder to grasp.
By writing this, I am now a persona non grata in the numismatic community. I promise, however, once I find my lost unicorn, I will tirelessly search for the mythical silphium.