Cheapest priced copy (The Lost Caverns of Ixalan): $0.35 at Card Kingdom, near mint, their price list as of .
Merfolk Cave-Diver 
Creature — Merfolk Scout · 2/4 · Uncommon · The Lost Caverns of Ixalan · #65
Whenever a creature you control explores, this creature gets +1/+0 until end of turn and can't be blocked this turn.
Legal inCommanderBrawlStandardPioneerModernLegacyVintage
Not legal inPauper
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| The Lost Caverns of Ixalan | 65 | Uncommon | $0.35 | $0.49 |
7 rulings from Wizards of the Coast
- 10 November 2023If an ability instructs a creature to explore, its controller reveals the top card of their library. If it's a land card, they'll put it into their hand. Otherwise, they'll put a +1/+1 counter on that creature, then choose to either leave that card on top of their library or put it into their graveyard.
- 10 November 2023If a resolving spell or ability instructs a specific creature to explore but that creature has left the battlefield, the creature still explores. If you reveal a nonland card this way, you won't put a +1/+1 counter on anything, but you may put the revealed card into your graveyard. Effects that trigger "whenever a creature explores" trigger as appropriate.
- 10 November 2023In some unusual cases, noncreature permanents may explore. For example, if the creature card returned by Defossilize is somehow not a creature once it's on the battlefield, it can still explore. You'll take all the same actions, and you may end up putting a +1/+1 counter on the permanent. (Note that some effects target a creature, and those effects would still require a legal target to have it explore.)
- 10 November 2023If a creature leaves the battlefield before an effect instructs it to explore, it still explores. Abilities like Merfolk Cave-Diver's that trigger whenever a creature you control explores will still trigger.
- 10 November 2023Once an ability that causes a creature to explore begins to resolve, no player may take any other actions until it's done. Notably, opponents can't try to remove the exploring creature after you reveal a nonland card but before it receives a counter.
- 10 November 2023Some spells or abilities might cause a creature to explore multiple times in a row. If you reveal a nonland card when a creature explores and leave it on top of your library, then the creature explores again immediately afterwards, you'll reveal the same card again.
- 10 November 2023If no card is revealed, most likely because that player's library is empty, the exploring creature receives a +1/+1 counter.