When you first enter the dungeon, you get the cheese and throw the switch that opens the back gate on the dungeon. (if you didn't, go back and do it) When you enter Morderd's secret underground lab, the wall to your left is just dirt, and you can push right through it.
You can't open the cell from the library entrance. Get the cheese and open the back gate by throwing the switch. Later you can get through the back entrance from Mordred's underground lab. (see related question)
After you enter the castle dungeon through the secret entrance in the library, all you do is throw the switch and grab the cheese. After that, the librarian and the guards won'y let you go back down. So to go back in, you get the key to the underground laboratory (beneath the hay bales by the mill). Once you get into it, there is only a dirt wall to stop you from reaching the back entrance to the dungeon cell. Push hard and the dirt collapses. (To get the mouse, key, and owl, see the related question.)
There are many doors on Astro Knights island, but none appear to be blocked. The gate to the dungeon cell is opened with the switch when you enter through the library. The underground lab is opened with the small key (from under the village fountain). The dirt wall is easily knocked down to access the back of the dungeon cell. Later in the game, the Golden Key must be drawn and the cards must be turned to open the entrance to Mordred's castle. (see related questions)
Big Nate IslandThe secret room is under the detention room, and can be reached through the filing cabinet.Astro Knights IslandMordred's secret lab is underground, with a locked entrance under the hay bales.(see related questions)
The fuel rod is in the alien invader in the dungeon. Enter through the secret underground lab : push down the dirt wall and make your way to the back gate, which was opened when you pulled the switch from the library entrance. (see related questions)
go to the dock and to the entrance where the shady guy is then go through the exit
You keep pushing until it finally collapses (it is dirt, not rock).
In the underground laboratory of Mordred, the wall to the left is not rock, just dirt. Push hard against it and it will collapse. (see related question)
In Mordred's underground laboratory, the wall to the left is just dirt and can be pushed down.
The trash can you find the Underground Tickets in is already open : it is the "right" one of the pair to the right of the Underground entrance. (Just paw through the refuse to the bottom.)
if you mean the one underground with the lock on the door then you have to be a super moshi then do pop goes the googoo mission
The wall in Mordred's underground lab is made of DIRT not rock, and with a little effort you can push right through it. The tunnel leads to the back entrance to the castle dungeon. (You should have already opened the gate when you threw the switch and got the cheese.) (see related question)