I just had an amazing experience. Well, I think it's amazing. I embarked onto a site with a relatively shallow set of levels, althought not by intent. The lowest is -52. I decided to build a spiral ramp down around the exact center of the site. Without much hassle I breached the first set of soil caverns at level -16, the water caverns at level -21, and the second set of soils caverns at -32. After setting up my farming and my well, and carrying out my first trade with the Mountainhome, I started down for the magma. At level -44 I encountered a warm wall while digging, and noticed that -43 also had a few warm spots. I went back up to the level above that one and expanded slightly (5x5) from the ramp. Nothing. I went back down a level, to -44, and started digging a couple of pilot 1-wide tunnels between the warm spots. I figured I'd start to get an outline of the magma sea that way. The first tunnelled tile gave me magma, adamantine, and the whole circus bit!
It turns out that this site somehow has the magma seas falling over edges into the circus levels. Magma-falls!
I quickly cancelled the other exploratory tunnel and ordered a door fitted into the existing breach. Then I ordered a two-layer deep wall built behind the door. Meanwhile the clowns were swarming around the back, intact wall of the ramp shaft, looking for the way into the fortress. Some, if not all, can fly, and if they'd had a more intelligent pathfinding they'd have had me. They would have had to duck down and fly over some shallow magma and up into the seas area on the side of the ramp shaft where my tunnel had breached the wall. They didn't seem to be able to do that, at least not in the time it took my Dwarves to seal the hole.
At this moment I have 74 clowns listed and I can't see how I will be able to access any of the magma safely. The one positive is that the four "ordinary" magma creatures are deceased.