The following is an excerpt from In the March and Borderland of Wales dated 1905 which describes an exploration of Porth yr Ogof.  Possibly the first description of a caving trip in Wales... - Allan Ockenden.

“It must have been nearly a half-mile walk by a grass path twisting about amid tall timber before we touched the further edge of the ridge and saw again just beneath us the bed of the stream. Scrambling down, we found ourselves confronted by the mouth of a great cave some twenty feet high, perhaps by forty wide, where the rocky bed of the river, evidently drained somewhere above of half its volume, vanished gradually into the gloom. Our programme did not include adventure in unknown subterranean passages, for my companion's intimacy with most things above ground in this corner of Wales did not extend to things beneath it.

But as we were regarding the capacious mouth of the cave with some interest, and meditating a little prudent exploration, a young man with a camera and a wife, apparently tourists, broke on our solitude. The former said he knew the labyrinths of the cave, and as he proposed penetrating them and had candles, he kindly suggested that we should accompany him as the trip was well worth making. L-- reminded me afterwards that it was I, not he, who at once accepted the invitation for both of us. At any rate we all entered the gloomy portal together, we two in our waders walking in the shallow stream, our obliging but amateur guides keeping dry feet by the exercise of a little agility on the stones.

For fifty yards or so, the natural tunnel was quite spacious, dimly lit by the fading light from its mouth. Now, however, a couple of candle ends were produced and ignited, for the water trickled away somewhere, and we had to face the Cimmerian blackness of narrow, tortuous passages, half blocked by huge boulders, over which we dragged ourselves, stooping more than once to squeeze between them and the roof of the cavern.

Once or twice we emerged into small chambers to vanish again through some magnified rabbit hole at one end of them, and crouch and scramble onward. It did just occur to me that I hoped we should get safe back again, but as the strange young lady had evidently no such misgivings I stifled the unworthy thought. All this time the candle ends carried by the strange young man and the misdoubting L-,- made but a feeble break on the oppressive and prodigious blackness. Our leader at length called a halt,.for which I, for one, was not sorry, and gave a caution which was still more needed. We were in a narrow chamber, if such a rugged cavity could thus be called, which ended in a black abyss from whose depths came the ominous gurgle of surging waters. We were then invited to lie on our faces and look over the edge, and there by the feeble light of the candles we could see, not far below, a black volume of water hurrying away into an inky void where not even a tallow dip could ever throw a flicker on it.

The complete gruesomeness of the spot was enhanced by the information that this Stygian flood was of a prodigious depth. In the cheerful bar parlour of the inn at Ystradfellte an hour afterwards we were assured it was bottomless. The air or lack of it grew oppressive, and after five minutes' contemplation-so far as contemplation was possible-of the creepiest place I have ever been in my life, we turned with some relief to scramble and wriggle out again. After a minute or two of apparently successful progress the wobbling candle of our guide suddenly stopped, and in a few seconds we were all together in one of those natural chambers such as we had traversed in our outward progress.

But this one had no outlet; we had taken the wrong burrow! We then floundered back, and we floundered about to little purpose; and I don't mind saying that for five minutes I experienced sensations of an entirely novel and most unpleasant description. An ordinary underground passage with a lantern is one thing, a boulder strewn rabbit burrow on a large scale with a couple of two-inch dips is quite another. Many unpleasant things flashed across my mind in a very brief period. Who was this stranger, on whose knowledge we depended for getting out of this accursed place? What if those two dips burnt out? A few matches; yes, but of what earthly use in a blackness where you could not see your hand placed within an inch of your eye.

The close air seemed more stifling than ever, and the whole of Wales seemed piled on the top of my head for the moment. I admired the coolness of the young woman, but she had doubtless more reasons for confidence in our guide than we had! Indeed, it was she who ultimately stumbled en the outlet that eventually brought us to the outer cavern and into the upper air again. Few remarks passed under-ground, for neither the method of progress, nor the little adventure coming back was conducive to conversation.

We then gathered from our guide that he had made the excursion only once before, and with much silent admiration for his nerve, and without, I hope, giving ourselves away, we bid the adventurous couple adieu, gathered up our traps, and started up a neighbouring lane that led eventually to Ystradfellte. Then we looked at each other, for we hadn't exchanged a word underground, each loth, no doubt, to admit the fear that had been within him.

Now my companion's worst enemies could not accuse him of want of nerve.

"Well! " said I at last, "how did you feel ~ "

"I felt," he replied, " as I never felt before and never hope to feel again. It was about the most disagreeable five minutes, certainly the strangest, I ever spent in my life."

I was much relieved at this, and comparing our sensations we found them to be almost identical, a horrible novelty being the prevailing note.

I should add that the main flow of the Mellte sinks underground some way above the cavern into which the partly filled channel we followed runs, and entering the hill at another point gives evidence of its course in the abysmal flood on which we gazed somewhere in the bowels of the earth.”