In the article the Slavic term * ǫb(ъ)lъ ‘source, spring; well’ is analyzed from the point of view of its word-formation and etymology. The discussion of preceding etymologies leads to the formulation of a new etymology, based on the internal analysis of * ǫ-b(ъ)lъ ‘source, spring; well’ as * n̥-bl̥o ‘unmuddy’ = ‘clear (water)’. The identified cognates in Baltic * balā, Germanic * pōla-, and possibly Continental Celtic * bolā, all ‘swamp, marsh’, imply the initial, ‘non-Indo-European’, * b-. Finally, it is tested, if the etymon could represent an extension in -l- from the root * gu̯ebh- ‘marsh, swamp’ in the zero-grade of ablaut. The answer is ‘yes’.