The eighth dungeon is labeled "TOP SECRET" on the map.
Open the Top Secret Building
With the treasure from the seventh dungeon, you will be able to reach the TOP SECRET building in the east half of the island. Push the block in front of the building to make holes appear in the ground. Once you fall in, approach the shadow over the water to make a Sharktula come down. Kill it to open the door to the left. Go through it.
Kill all the enemies you can reach (including the one that you can reach by breaking a rock in your path), then use the bottom right exit and lift a skull/shell, and kill the three skeletons in the corner. Go back to the previous room and use the bottom left exit (you have to swim) then in the next room break the rock in the way, and use the north exit, past where the rock was. Kill the Smileder to open the remaining door. Go south, then swim through the upper left exit, and then go left and go through where the door was.
Make your way to the next area. Don't stand on the cracked floor, and use the bow to shoot the target across the gap. Then return to the first room, go south, and go to the bottom left area of the water, where there is a rock you can break. After you break it, shoot the target on the right with the bow, and exit to the east.
Jump across the gap into the area where there is a skull/shell. Pick it up, then push up on the block that is above where the skull/shell was. Pick up the next skull/shell, then push down on the rightmost block. Go to where the closed door is, and jump to the right, across the gap, to kill the snails. (Or you can use projectile weapons to kill them from across the gap.) Go through the door you opened.
Kill the Smileder, then push up on the rightmost block, and push the lower block to the right. Go to the upper-left block (the one in the upper row) and push it to the right, then go around and melt the ice with the lighter, and push down on the block below that. Then talk to Maxwell. Go the way he went, push left on the block that is in the way, and go to the first room. Go through the door after Maxwell opens it.
After some dialogue, go to the right until you find Maxwell again. Stand in the middle of the +, say yes, and you will fall into the next dungeon.
Find the Treasure
The treasure of this dungeon is the Missinglasses. Use them near glitched tiles to reveal their true nature. Some glitched tiles can be walked through, while some contain heavy objects that you need to push on to lift, others may contain holes, or water tiles, and so on. You can also use the Missinglasses to wrap around to the other side of the screen while using them. Just stand near an exit, activate the glasses, and go through the exit to end up on the other side of the current screen.