In Exodus 20:4 there is a phrase בַּמַּיִם מִתַּחַת לָאָרֶץ that is often translated as "in the water under the earth". What location is this a reference to?
In fact here you have to merge two things. From the context you can understand that the writer refers to marine creatures and things. Translating Exodus 20:4 in modern terms: nothing coming from lithosphere, atmosphere and hydrosphere can become an idol.
Here is the point: what was hydrosphere for the writer? In fact wasn't a "sphere" at all. In the flat earth model we have water surrounding the perimeter of the lands, but we should consider even the hypothesis of water beneath the lands. In fact the term מִתַּחַת from base word תחת (ta-hhat) is a noun meaning "under," "below" or "lower." The prefix מ (me) means "from"1.
On this site you can find others biblical passages about waters under earth. Is quite obvious that ancient cultures thought about waters under earth: if you think that digging holes in the ground (e.g. building wells) many times they found water. So is possible that the authors thought that marine creatures, and others potential idols, could be in the waters beneath earth as well as in the seas.
1. Ancient Hebrew Research Center, Biblical Hebrew E-Magazine July, 2006, Issue #029