The Desert Underpass (Japanese: さばくのちかどう Desert Underground Tunnel) is a long tunnel located behind the Fossil Maniac's house on Route 114 in Hoenn. The passage is found only in Pokémon Emerald and is only open to Trainers who have entered the Hall of Fame.
Source
In Pokémon Emerald, after the National Pokédex is obtained, the player can go to the Desert Underpass through his house, where the unchosen Fossil can be obtained.
Also this:
After obtaining the National Pokédex
"Oh... It's not safe that way... I was digging away, you see. When the whole wall collapsed... I think there's a giant cavern underneath now... But I've left it alone because I don't think there are any Fossils there..."
Source for last two quotes.
The only game where there is an extension to the Fossil Maniac's house is in Pokemon Emerald. After you obtain the National Dex, the Desert Underpass will be available where you can find Whismer, Loudred, Ditto (which can't be found anywhere else in the game), and the other fossil you didn't obtain earlier in the game.
Source: Bulbapedia and experience.
