Mutating Pool

{t}: Add {c} to your mana pool.
Whenever one or more +1/+1 counters are placed on a nonland creature you control, put a +1/+1 counter on Mutating Pool.
{2}{g}{u}: Mutating Pool becomes a 0/0 green and blue Elemental Mutant creature with hexproof until end of turn. It's still a land.
A reservoir of progress.