One of the more common flavors of Planet Looters, in this case aliens are after one specific resource that Earth certainly has in abundance, water.
Involves some Fridge Logic as there is water in a variety of other more easily accessible locations in space. All comets and a variety of asteroids, moons, and even large, interstellar clouds have water in great abundance and are much more accessible and easy to harvest than Earth, as well as lacking those pesky hairless apes. Hence it rarely appears outside of Hollywood.
Ironically, using Mars itself as a setting for this Trope has become averted (barring specific details, such as potential toxicity levels or larger immediately available quantity) in Real Life as of September 2015 (NASA Link here).
- The Back Story of the anime Green Legend Ran is that giant wooden monoliths called "Holy Mothers" landed on Earth and drew all of Earth's natural resources into themselves - not just all the water but most of the breathable air. Travel between "Holy Greens" is essentially space travel, with airtight ships and suits. As one would expect, those who control the Greens control the water, and thus the population.
- Lampshaded in Plan 7 of 9 from Outer Space (along with a number of other B-Movie explanations) as to the motives of the alien invaders.
"The women will be held for ransom until we accede to the alien demands. That's it — they've come to steal our water!""But water can be mined from the Oort cloud or the Kuiper belt," said Proton. "They don't even have to land on Earth.""THEN THEY'RE DOING IT BECAUSE THEY'RE MEAN AND NASTY!" roared Zarkendorf.
- The people of Planet X's objective in conquering Earth in Invasion of Astro-Monster.
- In Battle: Los Angeles the alien invaders seem to be sucking up the oceans.
- Seemingly inverted in Oblivion (2013) where humanity is trying to drain the Earth's water as fuel for fusion in preparation for a Homeworld Evacuation. Until it turns out that the protagonist is a cloned slave unwittingly helping an alien machine drain the oceans and kill what's left of humanity.
- In the film adaptation of Ender's Game, Colonel Graff suggests that the Formics wanted to invade Earth for water, which their desert-like homeworld doesn't appear to have much of. Though they do attack a fleet of Formic ships harvesting ice from a gas cloud at one point. Note that this was not even discussed in the book, the theory there was that they were trying to colonize.
- David Bowie's character in The Man Who Fell to Earth is an alien who's come to Earth for water. In the brief scene we see of him at home with his family, they're all wearing skintight suits to keep their sweat in.
- While this isn't confirmed in-story during War of the Worlds (2005), director Steven Spielberg told Newsweek magazine in an interview that the aliens might be attacking for water.
- The Ice Pirates: While it's not an invasion that's the key to the movie, water is indeed the most valuable substance in the universe to the point of being the movie's MacGuffin.
- James S. A. Corey's The Expanse, it may not come from Earth, but the Space Cold War comes to a head because of water. The loss of a single ice freighter to military action from one set of absentee landlords is enough for the Belt to erupt into riots.
Miller: Belters don't take the long view when you screw with basic resources. That water was future air, propellant mass, and potables for us. We have no sense of humor about that shit.
- In Olaf Stapleton's Last and First Men the Martians invade Earth during the reign of the Second Men in order to steal water, along with plant life and diamonds. Then again it was written back when people thought Venus was covered with oceans of water (apparently too far from Mars) instead of lava.
- Downplayed in Animorphs. In the third book the kids see the Yeerks regularly send down supply ships to retrieve water and oxygen for their ships in orbit, but it's not the main reason for their invasion.
- In the Isaac Asimov novella The Martian Way spacecraft use water from Earth as reaction mass. To stir up anti-Martian sentiment as part of his campaign, an Earth politician named "Hilder" (though Asimov planned it as an attack on Senator McCarthy) says that spacers are using up Earth's water. In response the Martians go to Saturn and haul home one of the ice chunk asteroids which make up Saturn's rings, providing them with enough water to last 2000 years. The Martians snarkily offer to sell Earth some to "make up for" the minuscule amount of Earth water they've used over the years.
- The War of the Worlds: Global Dispatches. One character comes across a Martian staring rapt at a flowing stream, implying this trope as one of the reasons why the Martians invade Earth. In this case it would be a Justified Trope as the Martians are seeking to conquer a resource-rich planet for colonization, rather than just take the water back with them.
- In V (1983) the Visitors are after Earth's water, and the fact that humans are delicious too doesn't hurt. The Novelization has a good justification; in this Verse, the industrial effort of interstellar space development irreversibly destroys biospheres, and they have been unable to develop water purification technologies capable of efficiently supporting millions, let alone billions of people. Thus, the resulting empires are not only constantly fighting over whatever relatively pure water remains, but food as well; maybe they could harvest water from undefended comets(after filtering out twenty percent of their weight in ammonia), but a life-sustaining world which not only has over a quadrillion tons of fairly clean water but four and a half billion two-hundred-pound food animals too stupid to colonize space themselves? The planet looks like a buffet table guarded by illiterate street punks. And the Visitors have Frickin' Laser Beams... which happen to be fusion powered and need heavy water as fuel. note
- In the pilot episode of Star Trek: Voyager the Kazon-Ogla seek water because they're in a region of space without it, thanks to the environmental disaster inadvertently caused by the Caretaker. The crew of Voyager agree to trade water for information on the region, but unwisely explain that they can create water with their amazing replicator technology that the Kazon spend the next two years trying to get their hands on.
- One spotlight challenge on Face/Off required the creation of aliens whose home planets are in great jeopardy. Naturally, one of the planets is running out of water.
- The second scene of The Expanse has an OPA speaker claiming that Ceres was once covered in ice, but Earth and Mars have stripped it away leaving the Belters dependent on water shipments from Saturn. The loss of the ice freighter Canterbury to what seems to be a Martian attack spark riots throughout the Belt.
- Played with in Ricardo Arjona's song "Del Otro Lado del Sol" ("On the other side of the Sun"), which is the narration of an alien from a planet that has no water visiting Earth to see if it's viable to contact. He is from a race of Perfect Pacifist People, and is so horrified at how much Humans Are Bastards that he reasons going back to his planet to die of thirst is the better choice.
- A Calvin and Hobbes strip parodies this with a poem Calvin wrote about a flying saucer stealing the Earth's water and air.
- Downplayed in Half-Life 2. When close to some docks, if the player looks really close, they can notice that the dock isn't anywhere close to the shore, implying that there's less water in the ocean than before. Word of God also states that The Combine have placed a giant portal at the bottom of Earth's oceans, which is sending the water to other Combine-conquered worlds.
- The backstory of Alani from Battleborn involves this trope. Rendain decided to drain Alani's freshwater ocean planet of Akopos for a Jennerit reservoir fleet.
- In an episode of Doctor Snuggles called "The Remarkable Fidgety River", a race of aliens are stealing cubes of water from the ocean and taking them to their home planet. It turns out to be subversion, however, as the aliens meant no harm - they thought the water was rubbish because it had so much garbage in it. When Snuggles explains that the plants and animals on Earth need the water, the aliens promptly agree to give it back.
- The second season episode "When Strikes Mutilor" from Hanna-Barbera's SWAT Kats has a huge alien craft sucking huge columns of ocean water within sight of Megakat City. The aliens that normally man this craft are Technical Pacifists and are being held prisoner by the Space Pirate Mutilor, who has no qualms about disrupting Megakat City's ecosystem.
- An episode of Team Galaxy called "H2-Oh No!" featured an aquatic race stealing water from various planets because their own planet is in a system that has six suns, which constantly evaporate the planet's water supply.
- Steven Universe
- Played with in "Ocean Gem": Lapis Lazuli steals the Earth's oceans, but not for any chemical use—she wanted to get back to her home planet, and thought using her super-powerful hydrokinesis to build a Star Scraper was the best option. It still wasn't nearly enough.
- Inverted by the Gem Homeworld: They tried to colonize Earth for its minerals, physically consuming the planet's mass, but water seems to be the only thing they don't want. Homeworld has no rain and a graphic shows a fully colonized Earth would have no oceans, as it's not needed for their biology or technology—except perhaps a small amount used in Diamond Authority's "extraction chamber". They even appear to have created Lapis Lazuli's caste just to dispose of all that water.