ATLANTIS?

I dreamed I lived in an undersea bubble full of magical wonders and all sorts of magical people.

I was an explorer and a wizard.

I crossed the seafloor with my team of three fellow wizard-explorers and we happened upon a bubble-civilization we'd never seen before.

Entering the bubble, we shed our suits and soon became lost in a train station. We quickly met a man -- or rather, he met us. He appeared out of nowhere and began accusing us of intruding; we quickly explained our visit as explorers and, as we spoke, we realized that this bubble was entirely magic-less. The man was angry because we were beings of magic and had entered their bubble-civilization where magic was outlawed, not because we were foreigners.

He made us choose between taking a train ride for eternity or doing our best to leave, which, he warned us, would probably end in us dead.

We fought our way out, climbed back into our suits, and frantically swam back to our own bubble. Once there, we were told security had been breached somehow -- the protective shield that surrounded our bubble was still up, but an elusive saboteur was now wreaking havoc all around our bubble. After watching our security videos, my team and I noticed a blip on the screen that turned out to be the man from the magicless bubble, hidden by a Cloak of Invisibility and wielding magic.

As our response, we declared war on the other bubble. I went over to fight --

and then I woke up.