Mods that add new dimensions to the game typically require the player to build a portal out of a certain material or perform some special task to be able to enter these dimensions.

In Vertically Stacked Dimensions the way to unlock new dimensions is currently "just digging down".

Some ideas to allow mod-pack creators designing the progression more interesting:

• generating layers at certain heights that consist of blocks that require special tools to break and this way unlock the regions/dimensions behind them. (for ex. obsidian already exists as such a block)
• hazards that appear in certain height regions / dimensions. This may include mobs and/or bad effects & damage inflicted unless the player wears certain "protection equipment".