Some worlds possess a hidden core where life's essence constantly surges.

Mutavault

{T}: Add {C}.
{1}: This land becomes a 2/2 creature with all creature types until end of turn. It's still a land.

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{T}: Add {C}.
{1}: Mutavault becomes a 2/2 creature with all creature types until end of turn. It's still a land.

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{T}: Add {1} to your mana pool.{1}: Mutavault becomes a 2/2 creature with all creature types until end of turn. It's still a land.

Although Mutavault has all creature types while it's animated, it does not have the changeling keyword ability.

A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.