Unexplicably

William started to feel unexplicably hopeless.

Source: tatoeba (11605890)

Yuri became unexplicably ill after the trip.

Source: tatoeba (11886634)

[S]urely every encouragement ought to be given, and will be given to those professional men, or to any man who proves successful in throwing any light on this hitherto unexplicably dark and mysterious subject.

Source: wiktionary

And all these fatal novelties, spring, as from their germ, from the appalling necessity, not yet explained, by which the General Assembly of 1838—[…]—suddenly perished; leaving about one-fourth part of itself, to be its very self; while the remaining three-fourths perishing—not only survived, unchanged, their own destruction; but belieing^([sic]) this terrible metamorphosis, outlived the entity into which they had been so unexplicably recreated!

Source: wiktionary