hahaha yeah.. underneath the floor of the cargo area. Remove cargo mat (if equipped), remove false floor, remove black plastic bin and voilà , your battery.
Try to avoid licking both terminals at the same time. Same goes for the dogs. This gives you a bit of an idea: http://media.popularmechanics.com/images/10621_6.jpg Here here: http://www.autobeyours.com/gifs/05%20Prius...!%20005.jpg
Keep your dogs away from the terminals! Sasha wants to know if dog in the picture is a male it is too small! She woofed twice