Monumental Thirst! is an achievement in Halo: The Master Chief Collection. It is unlocked by finding and destroying the BLAST soda can on Remnant on Halo 2: Anniversary multiplayer. The achievement is worth 5 gamerpoints and can be unlocked in both matchmaking and custom games, though the latter is much more advisable.
The can is sitting in the water near the Albatross. Directly behind the stone arch, looking down into the sea, the player will see two rocks sticking up out of the water. The can is between the two rocks, next to the smaller one, just under the surface of the water.