relief well: how can they intersect with the other pipe, 5 miles below water level?

[davester]

Here's an industry link that discusses how directional drilling and drillstring navigation work: http://www.energyindustryphotos.com/how_...drille.htm

As far as intercepting another borehole, they often miss the first time. However, they don't have to start from the surface each time. They can pull back a ways and then start a new borehole that branches out from the original. Oftentimes there are quite a number of branches than come off of a single borehole.