Mossborn Hydra and Agatha's Soul Cauldron on the battlefield.
Fertilid and Crystalline Crawler in exile (exiled by Agatha's Soul Cauldron).
Mossborn Hydra has at least four +1/+1 counters on it.
({1} magic symbol) ({G} magic symbol) available.
Steps
Activate Mossborn Hydra using Fertilid's ability by paying ({1} magic symbol) ({G} magic symbol) and removing a +1/+1 counter from it, causing you to search your library for a basic land card and put it onto the battlefield tapped.
When the land enters the battlefield, Mossborn Hydra triggers, doubling the number of +1/+1 counters on it.
Activate Mossborn Hydra twice using Crystalline Crawler's first ability by removing two +1/+1 counters from it, adding ({1} magic symbol) ({G} magic symbol) .
Repeat until there are no basic lands in your library.
Results
Near-infinite colored mana.
Put all basic land cards from your library onto the battlefield tapped.