Vision Charm

[Kuma]

Suppose my opponent controls 3 Underground Seas as his only lands and I play Vision Charm on his upkeep choosing to turn all swamps into mountains. Can he still tap his Underground Seas for blue and do they still count as Islands?

[Dilettante]

Suppose my opponent controls 3 Underground Seas as his only lands and I play Vision Charm on his upkeep choosing to turn all swamps into mountains. Can he still tap his Underground Seas for blue and do they still count as Islands?

Okay, Vision Charm (http://ww2.wizards.com/gatherer/CardDetails.aspx?&id=3656) works by looking at all the lands in play of a particular subtype... and turning them all into a basic land-type of your choice. An Underground Sea (http://ww2.wizards.com/gatherer/CardDetails.aspx?&id=287) is a "Land - Island Swamp" according to Oracle. Therefore, if you name Swamp as the subtype and Mountain as the basic land-type, all lands with the Swamp subtype would become non-basic mountains... that are in play. If your opponent subsequently plays a swamp, it is still a swamp.

EDIT - Or to make it clearer, after you play your Vision Charm, if they play another Underground Sea, it is still... an Underground Sea. The others are still mountains.

[quicksilver]

To answer your question, underground seas in play would turn into non-basic mountains until end of turn. They will only be able to tap for red mana until end of turn.

[Nightmare]

"Underground Sea" is not a land type. Only swamp, island, forest, plains, mountain, locus, Urza's mine, Urza's tower, Urza's power-plant. I think that's it there might be one or two i missed. Basically land type comes after the hyphen on the Card type bar, it is not the name.
Lair, Desert, Urza's (see - Urza's Factory).

[Mulletus]

I think New jersey is a land type too. But you can't turn left, and if you get lost there they charge you $3 to leave.