Could a moderator please delete (or at least massively down-vote) the comment two above from squelettus? I'd argue that not only are they troublingly and confusingly wrong about needing Gxi where no "it" (subject nor object) was declared, as well as wrong about "there is" being the only literal translation of Estas, he/she apparently quit learning Esperanto some time ago, because his/her Duolingo profile lacks an Esperanto flag. Who knows, maybe it was because Esperanto didn't conform enough to his/her linguistic notions. Regardless, their komencanto level guesses shouldn't be immortalized here forever, confusing the impressionable, long after they stopped doing Esperanto on Duolingo.
Esperanto miri comes from Romanian mirare "wonder" -> Latin mīrāre "wonder" -> mīrōr "astonished" -> mīrus "astonishing" -> PIE *sméyrōs "laughing, smiling" (same origin as English smile)
I changed the etymology from Italian to Romanian because it seemed to fit better.
Esperanto doesn't quite have the same usages as English. In English it would be understood that you are describing something when you say "Amazing!" and therefore you would use an adjective. In Esperanto, unless you actually specify the noun that you are describing, you seem to use an adverb. I speak as a beginner and could easily be wrong.