Centuries ago, under siege from the Mycotyrant, the Oltec sealed the Core off from the world… until Huatli's party arrived.

Matzalantli, the Great Door // The Core

(Transforms from Matzalantli.)
Fathomless descent — {T}: Add X mana of any one color, where X is the number of permanent cards in your graveyard.

{T}: Draw a card, then discard a card.
{4}, {T}: Transform Matzalantli, the Great Door. Activate only if there are four or more permanent types among cards in your graveyard. (Artifact, battle, creature, enchantment, land, and planeswalker are permanent types.)

The Lost Caverns of Ixalan

Cards with the ability word "fathomless descent" have abilities that care how many permanent cards are in your graveyard.

Some descend triggered abilities include intervening "if" clauses (i.e. "if you have [four or eight] permanent cards in your graveyard" in the middle of the ability). Each of these abilities checks your graveyard at the moment it would trigger to see if it does. If you don't have the required number of permanent cards in your graveyard at that time, the ability doesn't trigger at all. If it does trigger, it will check again as it tries to resolve. If you don't have the required number of permanent cards in your graveyard at that time, the ability won't resolve and none of its effects will happen.

The Core's activated ability is a mana ability. It doesn't use the stack and can't be responded to.

Transforming Matzalantli doesn't cause it to leave the battlefield. It also doesn't cause it to become untapped.