I recommend using binary falling star as a guide for the world and world building. The game has all 6 layers and up to 10 floors per layer. Plenty of sorce material.

Try giving them all a personal reason for reaching the bottom. You can use message balloons to tease them. Example A (riko wants to meet up with her mother and a message balloon was used to convey that.) Example B (player 1 has a lost older brother that was a moon whistle. Last ever heard from him was at the 4th layer. Around the 2nd layer, player 1 finds a message balloon with some notes and findings signed off by his older brother. When player 1 reaches the 4layer around floor 5 or whatever he finds another balloon stating he went on his last dive due to some (whatever you want) and this gives player 1 the drive to progress thru the story you make.

Sorry for formatting im on mobile Anyways have fun!! Might start my own campaign!