Another example of a keyword that affects the cost of a spell and provides an alternative way to ease the cost of spells. The only time it has been used is in the Future Set block as a mechanic from some future set.

702.63. Delve

• 702.63a. Delve is a static ability that functions while the spell that has it is on the stack. "Delve" means "As an additional cost to play this spell, you may remove any number of cards in your graveyard from the game. Each card removed this way reduces the cost to play this spell by {1}." Using the delve ability follows the rules for paying additional costs in rules 409.1b and 409.1f-h.
• 702.63b. Multiple instances of delve on the same spell are redundant.

Rules Translation

• 702.63a. - When you're playing the spell, you can remove cards from your graveyard from the game and for every card you remove in this way it decreases the cost of the spell by one colorless mana.
• 702.63b. - Delve does not stack, it's redundant.