I guess I should elaborate on my reasoning for why I feel Dubious Depths may have had different BG's between acts. Metallic Madness at least has BG changes between its acts. It's most obvious in it's good future iterations; In act 1, it looks like a cross between a factory and a high tech greenhouse. But in act 2, it changes completely to an outdoor setting. Heck, act 3 also has BG changes, looks like you're closer to those purple buildings and octagonal.....things. I don't think it's outside the realm of possibility for Dubious Depths' BG and thus settings to change from a forested ruins area to a subterranean area going by this.