This topic comes up regularly.
In my 40 I didn't face the tank with sand to hide the dirt along the front and sides. You can clearly see the layers. The next time there's a snail next to the side under the sand into the dirt, I'll post up a pic.
On another member's thread (which I shamelessly hijacked, but apologized for!) I posted the same two pics as I did here, and Diana felt the snails would have no trouble getting past my cap to the soil. I'm watching youtube vids as well - they are mostly useless so far- one vid said how the snails decimated his plants! $100's worth, eaten up! lol Even though so many people see them as pests, and I understand they can reproduce fast and in great quantity, I think I will still try to get some. I believe that a greater biodiversity can only help my tank. Having the snails root under the cap will help my soil, their waste will fertilize my plants (I am strictly low tech) and hopefully they would get food missed by my fish on
the cap, before it adds to the ammonia levels (although I think my ramshorns do that). Speaking of which, one of the vids said they add a tremendous amount to the tank's existing bioload - so don't add them to tanks if you are concerned about the ammonia that the snails make. Confusing? Yep!