When loading my soon to be 8 year old world, moderate chunk(approximately 75x125) was malformed.

My house including various structures were affected. A ravine opened (pictured) under my house dropping passive mobs and destroying any blocks above the ravine. A lake was added in the south end of the area where a small river had been. Some areas were encased in a hill where none existed previously.

Sadly, my most recent backup was prior to a large building effort. So, I'll just live with the effect and backup more often.