The entire bottom is a semi-transparent red which allows you to see the innards. There are actually two red LEDs; the first one is located in the rear and casts a constant glow as long as the PC is turned on. It also lights up the entire bottom section. This is purely aesthetics though. The other light is for motion sensing and is located left of the hole. It's tilted and the stream is directed via a refraction mirror (this is speculation) so when the light bounces off the the underlying surface it goes directly up to the sensor. Also present on the bottom is the "genuine Microsoft" holographic sticker as well as the model number and some other useful information like the 5v/100mA electrical rating. The last interesting part is in the feet. Unlike some other mice I've seen which either have no footprints (Mitsumi) or dinky stickers which fall off a few months after usage (Logitech original scroll), Microsoft implemented solid plastic ones which are most likely super glued to the bottom. They also glide fairly well on most normal surfaces.