350 million years ago, there was a 20-foot tall mushroom in Saudi Arabia.

Called “Prototaxites,” the giant fossil was first thought to be a tree or some kind of algae. According to C. Kevin Boyce, a University of Chicago professor, a fungus was the last thing to be expected because “A 20-foot fungus doesn't make any sense... but here's the fossil.” Fungi are actually more closely related to animals than plants, but still have their own biological kingdom, as they are considered neither. 

This particular giant mushroom has now been found all over the world from 420 to 350 million years ago- when nothing but bugs and worms had made it onto land yet. What's even more interesting is that the tallest trees at the time were typically only three to four feet high, meaning this giant mushroom was most likely the largest living thing on land at the time. 

Though it would be really cool to see one of these in our time, Boyce says, “It's hard to imagine these things surviving in the modern world.” 

(Source)