Because she ate six pomegranate seeds.
Pomegranate.
Autumn-Winter
Because she ate the seed's of a pomegranate (the food of the dead) and Hades said she must stay in the underworld.
After Demeter would not let things grow and be harvested and the world would starve without her favor, Zeus sent word by Rhea their mother that Demeter would be let to have Persephone two thirds of the year.
Every year, Persephone spends the autumn/winter (when plants die) with Hades, and returns with spring/summer.
Persephone in Greek Myth had to stay in the underworld with Hades after eating food from it.
Some myths say that for one third of the year Persephone stays with Hades, and for two thirds of it with her mother.
Persephone ate of the pomegranate in the Underworld and thus had to stay in the Underworld for part of each year - during winder she was with Hades; during spring, summer and autumn she resided above the earth with her mother Demeter.
Yes the U-Boat can stay under water.
Persephone stays in the Underworld for half of the year (six months) and is allowed to return to Olympus for the other half.
Hades was the Greek God of the Underworld. He took Persephone (daughter of Demeter) as his wife (not by choice). So even though there technically isn't a goddess to rule the underworld, if you had to say there was one it would be Persephone.
Hades persuaded Persephone to stay with him in the Underworld by offering her a position of power and privilege as queen. He also reminded her of the benefits and pleasures she could enjoy in the Underworld, such as being by his side and ruling over the souls of the dead.