It's the 41st anniversary of Earth Day, which was created to raise awareness for environmental issues. A number of initiatives are undertaken on April 22nd each year around the world, to mark the occasion as well as make an actual impact.
The 2011 Earth Day doodle is particularly interesting one. It's a fully animated doodle, one of the few Google has done, though they are becoming more frequent.
All of the animals featured in the doodle, and there are quite a few of them, do something when you hover the mouse over them.
The two pandas near the bamboo trees act out one of the very first viral videos on YouTube, "The Sneezing Baby Panda" the small panda baby sneezes to the complete shock/surprise of the other.
Moving further to the left, a bird flies out of a tree, near the sun that stands in for the first 'o' in Google. Just below that, a lion isn't very happy about you pestering it with your mouse and isn't shy about showing it.
A penguin takes a dive from the glacier that makes up the second 'o,' straight into the river that is replacing the 'g.' Hover over the river and a salmon begins to swim upstream and up a waterfall to return to its birth place. Just when it's about to make it, it encounters a rather unpleasant surprise.
Near the top of the waterfall there's a koala bear moving up and down an Eucalyptus tree. A butterfly flies out of the bushes on the other side of the waterfall and a frog jumps into the water near the bottom of it.
The 2011 Earth Day doodle should be live in most, probably all, countries where Google Search has a localized presence. Last year, Google displayed one of the most beautiful doodles ever on Earth Day.