The Desert Underpass is one of the new Areas discovered in Pokémon Emerald situated in Route 115 Left of Fallarbor after you beat the Elite4

This is a long long barren tunnel that stretches across Hoenn, under Fallarbor and the Desert
Aside from the Wild Pokémon, it does have a further use. The Missing Fossil after Mirage Tower Crumbles lands right at the end of the Tunnel so that's a bonus!

Wild Pokémon in Desert Underpass

Here are the Pokémon you will find in the wild in Desert Underpass:

In The Tunnel
	Ditto	Common
	Whismur	Uncommon
	Loudred	Uncommon