The Kuiper Belt is a huge region of the solar system forming a ring around the sun just beyond the orbit of Neptune. It is much like the asteroid belt between Mars and Jupiter. The Kuiper Belt contains millions of small rocky and metallic bodies, but it's 200 times more massive. It also contains a large quantity of water, methane and ammonia ises, the constituents of cometary nuclei originating from there. The Kuiper Belt is also known for its dwarf planet occupant, Pluto and, more recently, fellow Pluto and Machia Machia. The Kuiper Belt is already a pretty unexplored region of the solar system as it is, but it has already thrown up something of a puzzle. The population of Kuiper Belt objects suddenly drops off at a distance of 50 astronomical units from the sun. This is rather odd as theoretical models predict an increase in number of KBOs beyond this point. The drop off is so dramatic that this feature has been dubbed the Kuiper Clif. We currently have no explanation for the Kuiper Clif, but there are some theories. One idea is that there are indeed a lot of KBOs beyond 50 astronomical units, it's just that they haven't accreted to form larger objects for some reason, and therefore cannot be observed. Another more controversial idea is that KBOs beyond the Kuiper Clif have been swept away by a planetary body, possibly the size of Earth or Mars. Many astronomers argue against this citing a lack of observational evidence of something that big orbiting outside the Kuiper Belt. This planetary theory however has been very useful for the doom sairs out there, providing flimsy evidence for the existence of Naviru or planet X. If there is a planet out there, it certainly is not incoming mail and it certainly is not arriving on our doorstep. So, in short, we have no clue why the Kuiper Clif exists.