This brilliant loop centers on Crystalline Crawler, which ordinarily trades its +1/+1 counters for mana on a one-for-one basis. However, Training Grounds slashes the activation cost of Kenrith, the Returned King's green ability down to just a single green mana, while Loading Zone doubles the counters you receive each time Kenrith resolves. This means spending that single green mana actually nets you a surplus of counters on Crystalline Crawler, allowing you to filter them back into mana endlessly. With this infinite colored mana engine secured, Kenrith, the Returned King can utilize its remaining abilities to draw your entire deck, gain infinite life, and resurrect every creature from all graveyards.