astounding

Of a nature to astound; astonishing; amazing; as, an astounding force, statement, or fact. -- As·tound·ing·ly, adv.

adjective

So remarkable as to elicit disbelief: amazing, astonishing, fabulous, fantastic, fantastical, incredible, marvelous, miraculous, phenomenal, prodigious, stupendous, unbelievable, wonderful, wondrous. See good/bad.

The adjective has 2 meanings:

Meaning #1: bewildering or striking dumb with wonder
  Synonyms: dumbfounding, dumfounding

Meaning #2: so surprisingly impressive as to stun or overwhelm
  Synonyms: astonishing, staggering, stupefying