The Bagger 288 is the world's largest tread-driven vehicle. It's 700 feet long and 300 feet high, and is a gigantic strip-mining machine. Yeah, it's a mobile dungeon level.
In the tradition of Thundarr, ancient relics have somehow not rusted out over the past 3,000 years, which means there's the occasional Gremlin or Pacer cruising around. Another lucky survivor is the glorious Bagger 288. It will be re-purposed by yet another mad wizard into a massive village-crushing wandering dungeon.
I won't spring this on the players for quite some time, treating this right requires some high level goodness. There need to be lots of useless fireballs and lightning strikes hurled at this thing before the players figure out they've got to scale it and get inside. Then, they'll have to fight off the sawblade-handed denizens within. When a machine has a several-hundred-foot-tall sawblade-looking attachment, that's got to be a theme. Everybody gets sawblades.