What is this movie called?

I have watched a movie that came out in the 80s or early 90s. It was about a prision that was really deep underground that was in the future I think. It had Sylvester Stallone in it and he was an undercover cop that went into the prision to try and get one of the criminals out. One of the criminals was innocent aswell. Also the prision was in a large sized circle and there was no fence or bars. It was an electronic thing that stopped them from escaping. The only way that they could escape was up to the surface. Please HELP me find out what this movie is called. I am so frustrated with finding it out on the net. It is not Fortress or THX 1138 or star wars.