Basement Key is an item in the “Plot advancement” category (Key Items).
In-game descriptions
| Ruby / Sapphire · Emerald | The key for NEW MAUVILLE beneath MAUVILLE CITY. |
|---|---|
| FireRed / LeafGreen | The key to NEW MAUVILLE, which was constructed beneath MAUVILLE CITY. |
| HeartGold / SoulSilver · Black / White · Black 2 / White 2 · X / Y · Sword / Shield | A key that opens a door in the Goldenrod Tunnel. |
Where to find it
Not placed on the atlas maps (so far: Red/Blue and Generation 3).
Sources : PokeAPI (items, item_flavor_text, machines) · atlas · pokeemerald species_info.h