Dig is a skill that allows a character to search for buried treasure. It may be used in the world map, towns, castles, dungeons, but not on ships.
Dragon Quest III
Appearing in the remakes, dig is learned by merchants at level 12. It can only be used five times per area and digging will normally excavate 1~5 coins, but there is a 1⁄128 chance to unearth 50% of the party's gold.
There is also a 1⁄8 chance to dig up an item as well, but only in areas where monsters are present. The item is randomly selected from the list of those carried by local enemies, meaning an item with a 1⁄256 chance to drop can theoretically be raised to an eighth. However, due to the hard limit imposed on the chance to excavate items in the first place this actually makes digging for items less efficient than stealing, and dig only appears in versions of III where thieves can join the party anyway.
Dragon Quest VI
Dig is learned by advancing to rank 3 of the Merchant vocation. It operates mostly the same as it does in III, but is no longer limited to five uses per area and the number of coins collected has been lowered to 1~2. Characters that use dig do not receive a bonus for being in the merchant vocation--the skill works the same regardless of profession.
Dragon Quest VII
A sea-dredging variant of dig was originally going to be a Hero only skill to further highlight the game's nautical themes, before being removed in the final version of the game.