Tinder fungus

Fomes fomentarius (commonly known as the tinder fungus), is a species of fungal plant pathogen found in Europe, Asia, Africa and North America. The species produces very large polypore fruit bodies which are shaped like a horse's hoof and vary in colour from a silvery grey to almost black, though they are normally brown. It grows on the side of various species of tree, which it infects through broken bark, causing rot. The species typically continues to live on trees long after they have died, changing from a parasite to a decomposer.
It was spotted growing on the side a cut tree trunk, in the forest near Makrirrahi village, Pelion mountain, Greece.