unter Tage

Etymology
lit. 'under day'. In the mining sense, refers to the (sun-lit) surface of the earth. In the temporal sense, is used in the sense of “during”, which is an Upper German idiom, except in the universally common phrase.

Adverb

 * 1)  underground, belowground, in a shaft or tunnel

Adverb

 * 1)  during the daytime