Cave-In is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

You may exile a red card from your hand rather than pay this spell's mana cost. Cave-In deals 2 damage to each creature and each player.

How it works : Cave-In

Original English text — Wizards of the Coast / Scryfall

No additional ruling has been published for this card.

Interactions with other cards

These cards' text mentions Cave-In.

Calamitous Cave-In

Calamitous Cave-In deals X damage to each creature and each planeswalker, where X is the number of Caves you control plus the number of Cave cards in your graveyard.