Where I live, the property owner is responsible for water lines under their property. My brother and my mother had to contend with this: the old water lines that go from the street to their houses leaked/burst, and my brother and my mother had to pay for the replacement since the leaks were on private property. Since it's happened to two members of my family within the span of a couple of years, I'm thinking I should sign up for the "insurance" offered by my water utility to cover such incidents. It's only $3 per month. But I'm lazy and have not done it yet.

It would seem to me that if the leak is on private property, then the property owner is responsible for fixing it. If the leak is under the street, then the water company would be responsible for fixing it. If it's under your neighbor's property, and it's not affecting them, but is affecting you since you are downhill, then the neighbor should fix the leak. Whether they will or not is a different question.

At least talking to an attorney would give you the facts of the law and then you could decide how best to proceed.