Monument of Depression is the second mission from Metal Slug 2 and X. It is the debut of the mummies.
This is the series first mummy mission. Right at the start, you will be attacked by Arabian berserkers. Kill them and proceed in the tomb. You'll see miners running out in terror, and soon you learn why. It looks like the whole place is infested with Mummies (or Dog Mummies in X). However, it's not a very hard stage. There is a vast number of weapons throughout the stage, so if you manage to avoid mummification, you'll never be in trouble. Keep in mind that the regular mummies can also easily be sliced earning you more points (the dogs aren't as easy).
Just advance through the first section of the tomb, picking up your Flame Shots and Shotguns (or Iron Lizard in X). In the second section, you'll see Rumi Aikawa right at the start. Soon after, you'll encounter an old corpse. Stab it for a 50000 point gem, then run away from its dynamite. After that, just continue your advance, picking the weapon of your choice to finish the mummies. Then climb to the tower to battle Aeshi Nero.
Metal Slug :
Absolute Thick Forest | A Wish For a Morning Glow | Iron Cavalrymen in Hell | Death Valley | The City Under Despotism | The Memorial Sea