I phase through the floor/ground quite a bit. The way I usually phase through the ground is just by imagining there's a hole in the ground and jumping into it. I use this as a reality check.

When I phase through the floor while indoors, I usually just end up in another room, nowhere significant to me at the time. Occasionally, I will end up in a duplicate of the room I started in. A couple of times, I have ended up in a fancy restaurant.

I've flushed myself down a toilet a few times. I usually end up going down a series of pipes. One time, I saw a dark underwater castle at the end of a large pipe.

When I phase through the ground, I usually end up someplace underground. One time, I phased through the ground in a park and ended up in a room where there was a ping pong table and a critter to play against (rat or rabbit, I forget which.)