Explorer Kit appears in 9 version groups since Generation IV, a Gameplay item in the Key Items pocket
PokéAPI item 405 of 2,223, Gameplay category, Key Items pocket
Effect, in PokéAPI's words
Explorer Kit: “Allows visiting the Underground.”
Sends the trainer to the Underground. Only usable outside.
Games
Explorer Kit is listed in the text of 9 version groups, first in Generation IV
Game index by generation
IIIIIIIVVVIVIIVIIIIX
Named in
Diamond and PearlPlatinumBlack and WhiteBlack 2 and White 2X and YSun and MoonUltra Sun and Ultra MoonLet's Go Pikachu and Let's Go EeveeSword and Shield