What is that?? Signs of the boogie-woogie aphid.

You're walking through the woods when you come across a tree with a dark patch underneath. It looks like maybe soot, ash, or perhaps spray paint?
Looking around at the bases of other trees, not a sign of the weird spot on any of the others, so you keep walking. ​

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You've just missed the sight of the boogie-woogie aphid (aka - Beech Blight Aphid).
Go back to the tree with the odd spot underneath (we'll wait), and look up.

You were so busy looking at the spot below you missed out on the great show up above! 
These aphids feast exclusively on beech trees gathering in massive numbers. They use their tiny proboscis mouth parts to drink the sap from the tree. Over all, they do very little damage to these trees - small branches or leaves may die but the trees themselves are fine. When you disturb the branch they're enjoying you will see how they got the name 'boogie-woogie aphid', their fuzzy bottoms will rise up into the air and they'll wiggle and pulsate menacingly at you. 
So what is the stuff all over the ground?
Well, with the feeding frenzy up above, the aphids excrete a honeydew (aphid poo) that builds up under the tree. When it builds up, the fungus (Scorias spongiosa) Sooty Mold Fungus, works to break it down, so what you noticed first was the fungus growing on the honeydew that was dropped by the aphids above.
​Nature is so cool.