;D Sound familiar !! we had one plot that had a HUGE mound in the middle !! no one knew what it was and when i started to dig it i found over 200 whole eggs, several old boots, toy cars, ending up witha 10 x 8 shed at the bottom ;D it was an experience but i dont think you can get a new shed from the remains of an old one :D
We found out a lady used to feed the foxes eggs and they buried them in the pile !! had to be VERY careful not to break them as they stunk :P
Keep going and work back to the bad bit later, get something in the ground to give you a boost :)