Continent Mammoth

Continent Mammoth enters the battlefield
tapped.
Continent Mammoth enters the battlefield with {x} +1/+1 counters on it.
Continent Mammoth doesn't untap during your
untap step.
Remove a +1+1 counter from Continent
Mammoth: Untap Continent Mammoth.
{b/g}{b/g}, {t}, Sacrifice a land: Add {x} mana
of any one color that land would produce, where {x} is Continent Mammoth's power.