Underwater Tunnel // Slimy Aquarium

The Room split wires card selection and board development to opposite halves of the same permanent, and the two doors ask for wildly different amounts of mana. Pay a single blue to unlock Underwater Tunnel and surveil 2 sifts the top of the library while stocking the graveyard; pay the heavier cost to unlock Slimy Aquarium and manifest dread drops a face-down attacker that lands with a +1/+1 counter already on it, a hidden body you can flip later for its printed identity. What makes the split coherent is that the halves never compete for the same turn. The Room structure lets the mana spread across the game: spend the cheap digging side early as a smoother, then hold the threat side until the mana is spare, unlocking either door on the battlefield whenever the turn allows. Both halves feed a graveyard-and-face-down blue plan without either one carrying a card on its own. The surveil sculpts a self-milling deck's yard and clears clutter off the draw; the manifest dread half converts that same excess into a growing threat, turning cards you would otherwise dig past into a 3/3 with upside. Selection and board presence rarely cohabit a single permanent this cleanly. A patient blue deck buys information when it needs it and a body when it can afford one, from a card that unlocks itself in whatever order the game demands.