Mt. Doom Reconnaisance
As you move along the tunnel, you don't detect any secret doors, no concealed doors, no creatures burrowing beneath you, nothing trying to sneak up on you, and no hidden traps.
The insect makes it about a foot into the cavern before it burns up.
It would seem that the farther you go into the cavern, the greater the heat gets. There's probably an upper limit, though, and it's probably not as hot as the magma. Then again, the heat would seem to be coming from the magma.
As Stein studies the cavern, he realizes that the magma is being kept out of the cavern by (probably) walls of force. And, yes, the magma through the holes is most likely the lake of magma from the surface.
It's hard to determine if the magma is magical or if the magic is just the wall of force keeping it out.
The creatures below all seem to have a magical aura to them, but it's unclear what it is, other than to say it does not appear to be conjuration. The two exceptions are the fire giant who seems to have several magic items on him, including huge axe, and the three silvery men walking about. At the very least, they have wands that they point to the walls, floor and ceiling at various times. There are also more magmin down here, plus fire elementals, salamanders, and hellhounds. It's the latter that concern you, for they cast eyes upward to the tunnel now and then, as if they might be sensing something... perhaps the buzzing of the insects.
You're really too far away from the creatures below to get any sense of what they are saying. In any case, you don't pick up any indications of portals from them.