shh....
my secret 3CB tech.

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Ice Floe
Color= Land Type= Land Cost= None IA(U)/5(U)
Text (5th): You may choose not to untap ~this~ during your untap step. ;

: Tap target creature without flying that's attacking you. It doesn't untap during its controller's untap step as long as ~this~ remains tapped. [Oracle 2000/01/15]
What makes me wonder is the ambiguity of "It". I believe that it does work for manlands as I believe that it refers to the card itself, not the creature. (or else I wouldn't have submitted it). But I'm not 100% sure so wait for Jebus or Jaap