Hidden Cataract

The trick with any land that also does something is amortizing the cost, and this one hides its price across the whole game. The mana ability is unremarkable: a tapped source of blue, the tax every dual and utility land pays for its second function. What that tapped land buys is patience. It sits in the manabase producing blue for as long as you need it, and then, once you can spare five mana from other sources and it is your main phase, it converts itself into Discover 4: exile from the top until you hit a cheap-enough spell, then cast it for free or bank it. The sacrifice clause is what keeps the rate honest, since you are trading the land itself for the effect, not stapling a card-advantage engine onto an untapped blue source you keep forever. It is a topdeck you schedule in advance and pay for across many turns rather than all at once. The design belongs to a broader idea of caves as lands that graduate into spells late in a game, when the mana they were providing matters less than the card they can find. The floor is a slightly slow blue source; the ceiling is a free spell off the top once the game has run long enough that a land no longer needs to be a land.