This is a classic case of a Dandy Walker malformation.  The most severe end of the Dandy Walker spectrum.

• hypoplasia of the vermis and cephalad rotation of the vermian remnant
• cystic dilatation of the fourth ventricle extending posteriorly 
• enlarged posterior fossa with torcular-lambdoid inversion (torcular lying above the level of the lambdoid due to abnormally high tentorium

In addition in this case there is gross hydrocephalus as a consequence of the posterior fossa abnormalities, with periventricular high signal in keeping with trans-ependymal edema.