Snowball Earth or Icehouse Earth refers to times when the Earth's surface was nearly or entirely frozen. The occurrence of Snowball (or Slushball) Earths is still controversial, but it is now probable that widespread glaciation occurred in periods of the Proterozoic. What is still debatable is how widespread those glaciations were. Proponents claim the theory explains sedimentary deposits of glacial origin at tropical latitudes and other enigmatic features of the geological record. Opponents do not draw the same inferences from the geological evidence, and doubt the geophysical feasibility of an ice or slush-covered ocean.