Underground Sea

a Swamp enters the battlefield — 63 cards in the game can cause this (Underground Sea is one).

When Underground Sea enters:

1. Thelon's Chant — deals 3 damage
2. Veilborn Ghoul — returns itself from the graveyard
3. Staff of the Death Magus — gains you 1 life
4. Dread Presence — triggers

an Island enters the battlefield — 64 cards in the game can cause this (Underground Sea is one).

1. Staff of the Mind Magus — When Underground Sea enters, Staff of the Mind Magus gains you 1 life

a land enters the battlefield — 1,542 cards in the game can cause this (Underground Sea is one).

When Underground Sea enters:

1. Primeval Bounty — gains you 3 life
2. Dancing from Dark to Dawn — makes a token
3. Toph, Earthbending Master — puts a counter on a permanent
4. Emeria Shepherd — brings a card back
5. Quest for the Necropolis — puts a quest counter on itself
6. Soul Stair Expedition — puts a quest counter on itself
7. Nissa of Shadowed Boughs — puts counters on a permanent
8. Thranduil, Sindarin Liege // Silvan Rally — makes a token
9. Silvan Reveler — returns itself from the graveyard
10. Floral Evoker — puts a +1/+1 counter on itself

190 other cards care about it too. These are the ones with the most connections.

anything enters the battlefield — 26,290 cards in the game can cause this (Underground Sea is one).

1. Nature's Wrath — When Underground Sea enters, Nature's Wrath makes a player sacrifice an Island or permanent

17 other cards care about it too. These are the ones with the most connections.