I know cosmasad has just thanked us, but going back to cosmasad's original post, he was looking for an equivalent greek phrase for "an underground stream of water that comes out in to a spring"... we're missing the feeder function of this underground stream (what cosmasad described was not just an underground stream, which may or may not emerge at the surface - it's an underground stream that feeds a spring or flows into a spring, in fact).
For this maybe we need something like ... υπόγειο τροφοδοτικό ρυάκι, υπόγειος ποταμός που αναβλύζει ως πηγή, or similar?
(with φλεβά νερού I just wanted to convey the feeder aspect - a hidden vein that feeds the spring. I realise it's colloquial and lightweight).