The Underground Path is a tunnel linking two areas in the Kanto region in the "Pokemon" video games. The Underground Path runs between Route Five and Route Six underneath Saffron City, with the entrances at the north and south ends of the city. There are no wild Pokemon in the tunnel, but several items are hidden there that can be uncovered by scanning the relevant areas with the Itemfinder tool available from Ecruteak City.
In "Pokemon Red," "Blue" and "Yellow" versions, a Full Restore is found two floor tiles to the left of the top stairs in the Underground Path. In "Gold," "Silver," "Crystal," "HeartGold" and "SoulSilver," the Full Restore is hidden in the northern part of the tunnel. A Full Restore is a healing spray that recovers all of a Pokemon's Health Points.
In "Red," "Blue" and "Yellow" versions, an X-Special item is hidden next to the left wall, 32 tiles down from the top of the tunnel. In "Gold," "Silver," "Crystal," "HeartGold" and "SoulSilver," the X-Special is in the right corner of the path. An X-Special is an item you can give to a Pokemon that increases the holder's Special Attack rating by one point.
In "Pokemon FireRed" and "LeafGreen" versions, a Burn Heal is hidden near the bottom of the tunnel, two tiles up from the stairs, and two tiles to the right of the left-hand wall. A Burn Heal is a status healer which cures a Pokemon of a Burn suffered in battle -- if left untreated a Burn will remove Health Points from the Pokemon every turn.
You can get the Technical Machine for the move Explosion in the Underground Path in "Pokemon HeartGold" and "SoulSilver" by trading the RageCandyBar with the man in the tunnel. You can buy the RageCandyBar from the man at the western exit of Mahogany Town -- it is a Key Item, and its only use is to trade it for TM64.
It is possible to come across other items randomly in the Underground Path by scanning with the Itemfinder. Items that appear around the tunnel include Antidote, which cures a Pokemon of poisoning; Awakening, which wakes up a sleeping Pokemon; Ice Heal, a status healer that revives a frozen Pokemon; Paralyz Heal, which cures a paralyzed Pokemon; and Potion, which restores 20 health points to a Pokemon.