Thus somewhat of the depth was revealed to us, and there seemed something peculiarly awful to me in the fierce glowing red heat of the shores themselves, which never cooled with exposure to the air and light.
Thus acted the first of the two lakes.

But when, favored by a strong breeze, we ventured farther, to the side of the furthermost one, a still more terrible spectacle greeted us.

The mass in this lake was in yet more violent agitation; but it spent its fury upon the precipitous southern bank, against which it dashed with a vehemence equal to a heavy surf breaking against cliffs.

It had undermined this lava cliff, and for a space of perhaps one hundred and fifty feet the lava beat and surged into glaring, red-hot, cavernous depths, and was repelled with a dull, heavy roar, not exactly like the boom of breakers, because the lava is so much heavier than water, but with a voice of its own, less resonant, and, as we who listened thought, full of even more deadly fury.
It seems a little absurd to couple the word "terrible" with any action of mere inanimate matter, from which, after all, we stood in no very evident peril.

Yet "terrible" is the only word for it.