Advice on Cleaning a Nasty Inground Pool

We did not open our inground pool last season due to a health issue of a family member. It stayed covered up all summer and winter and we just removed the cover. The bottom is filled with a sort of "goo" of rotting leaves and other organic material. Most of it slides right through the skimmer bag thing I run along the bottom.
Anyone have any tricks or ideas on how to get this junk out of pool? Most of it does NOT float and sinks to bottom after being disturbed. For reference our pool is about 20' by 40' and approx 25,000 gallons.