A Cottonwood with a big crack in the base caught my attention.
I looked inside and found the following mushrooms.
These mushrooms are probably a kind of polypore, a mushroom with pores under the cap, instead of the more familiar gills. There are a lot of different kinds of polypores, but they can hard they can be hard to ID (like most mushrooms really). Here's a discussion on polypore ID from Tom Volk's website (a favorite of mine), and another one from Michael Kuo's Mushroom Expert website (another favorite)
Here's another picture "looking" up into the hollow space of the tree (I couldn't fit, I just held the camera and pointed up).
The white area is probably a spot where a fungus that degrades lignin has been at work, leaving behind white cellulose (maybe the fungus I was looking at). Lignin and cellulose are both components of wood. Lignin is only degraded (or "eaten") by a few groups of organisms; bacteria (they can do anything, can't they), fungus (polypores being a big one), and possibly some insects (but possibly only with the help of bacteria or fungus in their gut). The mushroom that we see is just a part of the organism, with most of the fungus doing the work of living inside of rotten wood, living trees, in the ground, etc. In a way, I'm surprised that there aren't more groups of organisms that can break down the various components of wood, given how widespread trees are, but maybe that has something to do with the fact that trees are so widespread. But luckily (or whatever) there are bacteria, fungus, and maybe insects that can do it; they recycle nutrients into the soil, play a role in the carbon cycle, and contribute to the beauty of our world (I like rotten logs).
Anyway, here are a couple more photos of the rotten life of this tree
Now go forth and enjoy your own rotten tree!