Seriously -- find a nice loud speaker that will fit in the exposed end of the pipe. Block the exposed end to muffle the sound as much as possible.

If you have a mechanic's stethoscope, great! If not, a piece of steel (like rebar) will work by placing one end against your ear. Start near where you think the other end is, then move farther and closer to determine if you're getting nearer to the end of the pipe. If you're really lucky, you'll touch the pipe itself and get a solid sound. If not, you should be able to narrow the search area quite a bit.

I'm assuming you can't see which direction the pipe is going,or that it's curved -- otherwise you would have lined up a forty foot tape measure on the surface.