Making non-colliding prims

OK, I'm trying to make a pond in a high-ground location (way, way above sea level), so I kind of need to know this. Maybe I missed something blindingly obvious, but here goes anyway:

How do I make it so people will walk THROUGH a water prim rather than ON TOP OF it? In other words, I've textured the prim as water, but there's no "water" material to let people walk through it. (Not as far as I noticed, anyway.) Is there a way to make this work?

Thanks in advance.

- A.W.

Tick the phantom box in the Object tab

D'oh. *smacks forehead* Thanks.