When a farm servant was once lying in hiding near the Ynys Geinon Rock, waiting for some perverse rabbits to enter his net, he saw a little man going up to that great mass of stone. On his uttering a curious little word, a door opened in the face of the rock: he went in, and the door closed behind him.
Dai (every other man in South Wales has this pretty name) thought he would see what would happen if he uttered the same little word as the little man had used. He tried the experiment: the door opened for him also, and he went into the rock. But he could not shut the stone door behind him, and when he saw that it weighed at least three or four tons, he did not want to do so. At this juncture a little man came running towards Dai shouting, ‘Shut the door, shut the door, the candles are guttering with the draught.’ With that he uttered another curious little word, and the door shut of its own accord. Then he noticed the intruder and called his companions. They made great sport of Dai, but as he was ruddy and of a fair complexion they treated him kindly.
He found that there were underground passages running in all directions: they could get to the Cave of Tan yr Ogof, near Craig y Nos Castle, the Caves of Ystrad Fellte, the Garn Goch, and other places by them. He learned, too, much about their habits: these fairies were dreadful thieves, always stealing milk and butter and cheese from farm-dairies.
After he had been, with them for about two years they let him go, and gave him a hatful of guineas to take with him, for they had great stores of gold. He told his master all about his experiences when he returned, but it would have been better if he had kept his knowledge to himself. His master thought it was a great pity that so much gold should lie idle, and opening the stone door by means of the password which Dai had learned, he brought from the cave enough guineas, half-guineas and seven-and-sixpenny pieces to fill his salt chest. But he became too greedy; and when he went, to the cave to fetch still more money, the fairies caught him, and he never returned. When Dai went to look for him, he found his four quarters hanging behind the stone door: he was so frightened that he never again ventured to use the password, nor would he reveal it to anyone, so that this very useful bit of information has perished, which is a very great pity (Thomas 1907 159-160).