Genesis 1:20 reads:
וַיֹּ֣אמֶר אֱלֹהִ֔ים יִשְׁרְצ֣וּ הַמַּ֔יִם שֶׁ֖רֶץ נֶ֣פֶשׁ חַיָּ֑ה וְעֹוף֙ יְעֹופֵ֣ף עַל־הָאָ֔רֶץ עַל־פְּנֵ֖י רְקִ֥יעַ הַשָּׁמָֽיִם׃ (WLC)
Which translates as:
And God said, “Let the waters swarm with swarms of living creatures, and let birds fly above the earth עַל־פְּנֵ֖י רְקִ֥יעַ of the heavens.” (ESV)
The phrase עַל־פְּנֵ֖י רְקִ֥יעַ, literally something like "on the faces of the expanse," seems to be especially difficult to translate without referencing some outside cosmology since it doesn't make a lot of sense literally. (To clarify, when I say literally, I am thinking of "face" as the surface of something solid. E.g., walking on the face of the earth. Thinking of something like "fish swimming on the face of the waters" will perhaps clear up why I have trouble understanding the phrase literally.)
The King James chooses "in the open firmament" which is updated in the NASB to "in the open expanse." However, other most translations render is rather differently. The ESV and NET have "across the expanse" while the NIV has "across the vault [of the sky]" and the "across the dome [of the sky]."
Presumably ancient people has eyes and could see birds (or "flying creatures") flew up and down and thus not on the "face" (surface) of a possibly firm barrier that divided the waters (see Gen 1:6-8). As such, the phrase, it seems, must be idiomatic, as all translations take it. But, what clues do we have as to how the phrase is being used? I would prefer an answer that doesn't presuppose the account backs a certain cosmology - either the modern one of the "typical" Ancient Near East one. (I would also consider an alternate argument that the phrase is using "poetic license" and thus not an idiom per se.)