‘Tis not every Soyl that is fit fewel for the fire.

“The difficulty, no doubt, will be chiefly from the great quantity of water that is about our Globe; whereby Nature seems to have made provision against any invasion by fire, and secur’d us from that enemy more than any other. We see half of the Surface of the Earth cover’d with the Sea’s: whose Chanel is of a vast depth and capacity. Besides innumerable Rivers, great and small, that water the face of the dry Land, and drench it with perpetual moisture. Then within the bowels of the Earth, there are Store-houses of subterraneous waters: which are as a reserve, in case the Ocean and the Rivers should be over-come.

Neither is water our only security, for the hard Rocks and stony Mountains, which no fire can bite upon, are set in long ranges upon the Continents and Islands: and must needs give a stop to the progress of that furious Enemy, in case he should attack us.

Lastly, the Earth it self is not combustible in all its parts. ‘Tis not every Soyl that is fit fewel for the fire. Clay, and Mire, and such like Soyles will rather choak and stifle it, than help it on its way. By these means one would think the body of the Earth secur’d; And tho’ there may be partial fires or inundations of fire, hence and there, in particular regions, yet there cannot be an universal fire throughout the Earth. At least one would hope for a safe retreat towards the Poles, where there is nothing but Snow, and Ice, and bitter cold. These regions sure are in no danger to be burnt, whatsoever becomes of the other climates of the Earth.

As to the Central Fire, I am very well satisfied it is no imaginary thing …  And tho’ I do not know any particular observation, that does directly prove or demonstrate that there is such a mass of fire in the middle of the Earth; yet the best accounts we have of the generation of a Planet, do suppose it; and ‘tis agreeable to the whole Oeconomy of Nature; as a fire in the heart, which gives life to her motions and productions … This Central Fire must be enclos’d in a shell of great strength and firmness; for being of it self the lightest and most active of all Bodies, it would not be detained in that lowest prison without a strong guard upon it. ‘Tis true, we can make no certain judgement of what thickness the shell is, but if we suppose this fire to have a twentieth part of the semidiameter of the Earth … there would still remain nineteen parts of the semidiamater of the Earth will make a partition-wall betwixt us and this Central Fire.”

Thomas Burnet – Sacred Theory of The Earth (1681)

Quoted in Why Hell Stinks of Sulphur.