Sub Rebellion, known in Japan as U: Underwater Unit (-U- アンダーウォーター ユニット, -U- Andāwōtā Yunitto), is a submarine simulation video game developed by Racjin and published by Irem and Metro3D for PlayStation 2 in 2002.

In the present, the Meluguis Empire, a cruel, oppressive and militaristic regime, is striving for world domination, and it is opposed by the Allied Forces.

The second mission involves fighting through an ancient ruin to reach a goddess statue buried under rock, while surviving an enemy driller submarine ambush after the excavation.

The fourth mission involves deployment to the Caribbean, fighting to destroy multiple unmanned enemy underwater batteries positioned along and inside a massive fissure, followed by rescuing an allied submarine trapped under fallen rocks.

After this rescue, the Chronos is immediately attacked by the Hammerhead, a very fast and technologically sophisticated submarine crewed by mysterious enemy mercenaries in service to the Empire.

The fifth mission involves fighting through a cave in close-quarters combat to eliminate Imperial submarines and excavate a strange artefact (called Medusa Head).

The seventh mission is a deployment to the southern tip of Greenland, involving the neutralisation of three heavily defended Imperial command centres of land bases.

The fourteenth mission is a deployment to an ancient ruin, involving protecting three allied submarines equipped with special sonar against attacking enemy forces.

The sixteenth mission is a deployment to the old city area, involving eradicating an enemy special weapons team prepping an extremely dangerous device.

The nineteenth mission involves hunting down and wiping out two large Imperial fleets attempting to flee to the Pacific Ocean.

In the twenty-first and final mission, the Chronos crew are tasked with investigating the Promethean Holy Ground and discovering the "Stillness".

The charters reveal that the High Council, the leaders of the Promethean Civilisation, ordered the deep submersion of the majority of the Earth into water.

The charters reveal that the Prometheans are aliens, and that the vast majority of them evacuated the planet, returning to the Mother Star before Earth's submersion, with a small number of them choosing to remain.

Its crew laments its failure to protect the Holy Ground, before they commit suicide by smashing their vessel into the central energy barrier, destroying it.

In the epilogue, it is confirmed that a geosphere manipulation device found in Promethean Holy Ground was used to cause the cataclysmic submersion of the Earth.

It is stated that researchers believe that, while it may take time, the water levels will drop and the Earth will eventually return to normal.

The game asserts that that technological advance walks hand-in-hand with great tragedy, and that countless battles in the name of noble causes show this all too well.

The game also asserts that if humanity is ever to outgrow the fundamental drive to war, it must progress to a more advanced level of consciousness.