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Buy a map at the shop
Or kill cartographer !
Every map (obtained by cartographers or bought in the shop) has a cartographer in it that drops another map to the next level (signaled by the green dot), possibly going beyond the normal dungeon floors cap. Fastest thing after completing the normal levels is to buy level 200 map from the shop (after selecting the corresponding level) and then proceed killing cartographers from it on.
just be aware that the higher you go, Maps from the Shop get more expensive. so buy a spare map before you log out. also, save a Map for floor 500 when you get there, as that is a good floor to Farm for Ultra Rare Crystals & Myth Stones.