El Dorado, City of Gold

{t} Add {u} to your mana pool.
Tap an untapped merfolk you control: Add 1
Exploration counter to El Dorado.
When El Dorado has 5 or more Exploration
counters on it, it has {t} add {g}, {w}, {u} or {b}
to your mana pool, and "merfolk creatures you
control are unblockable."