Since there’s surprisingly no Russian here yet, here are a few I know (some are probably regional):

• Где раки зимуют [Gde raki zimuyut] - “where crayfish spend their winter”
• У чёрта на куличиках [U chyorta na kulichikah] - “on the devil’s little Easter breads” (there’s also a version “on the devil’s horns”)
• За тридевять земель [Za tridevyat zemel] - “beyond threenine (i.e. 27) lands” feels somewhat fairytale/archaic, haven’t heard anyone use it unironically
• В жопе мира [V zhope mira] - “in the ass of the world”
• Зажопье [Zazhopye] - a made up placename roughly meaning “beyond ass”, evocative of many toponyms which start with the same prefix (e.g. Замосковоречье [Zamoskvorechye] - “beyond river Moscow”)
• Мухосранск [Mukhosransk] - made-up town name, roughly translatable as “where flies shit”
• Ухрюпинск [Ukhryupinsk] - corrupted name of the town of Uryupinsk, probably on account of it being far away from my birthplace
• Подзатворожинск [Podzatvorozhinsk] - made-up town name, roughly “[place which] started to turn into cottage cheese” or “under the [place] beyond cottage cheese” or some combination of the two

But honestly the most likely phrase I’d use in an informal context is a boring чёрт знает где [chyort znayet gde] - “devil knows where”