Where is Mesprit in Pokemon Explorers of Darkness?

You first meet the legendary Pokemon 'Mesprit' in the deepest part of Quicksand Pit, where he is guarding one o the Time Gears.

Quicksand Cave becomes accessible after you meet Dusknoir and learn of the plot to steal Time Gears. Dusknoir composes a list of possible locations for the Time Gears, including the Northern Desert. You and your partner set out to explorer Northern Desert, however when you reach the end, you find nothing but several quicksand pools.

Later, on an impulse you and your partner return to the pools of quicksand, and on an impulse, leap into the largest. Through here you find yourselves in a new dungeon, Quicksand Cave.

After here it's pretty straightforward, you get to the end find Mesprit, who battles you because he thinks you want the Time Gear. However, by the time he trusts you, Grovyle appears and steals the Time Gear, forcing you to flee the cave before time stops completely.

You can later return to Quicksand cave to battle and recruit Mesprit, but only after you have completed the storyline.