They were determined to invade the curious dwelling place of the natives. It was a moral certainty that the white prisoners, Lucille and Mark Vane and Alvan Bates were therein confined.
This being the case, there was sufficient excuse for the invasion, for it was necessary to rescue them.
The airship descended until on a level with the cavernous opening. It could easily have sailed into the place, but Frank was afraid that collision with the roof might damage the wings or rotascope.
So he did not venture to enter.
But getting down on a level he turned the rays of the searchlight into the place. This revealed a curious sight.
A mighty open space, or perhaps it might be called cavern, occupied several acres in extent, and all roofed by the shell of the volcano.
But in the centre of this vast underground area was what looked like a lake of molten gold as it lay under the gleam of the searchlight.
However, Frank saw that it was nothing of the kind, but a vast basin of boiling lava.
A stream of the boiling liquid ran down into the basin from an orifice in the mountain wall.
The walls of the immense cavern were of hardened lava, apparently. It was certainly a queer freak of nature.