"There is another wonderful thing in the area which is called Guent. There is, in that place, a pit, from which blows out a wind the whole time without intermission. And when no wind blows in the season of summer, from that pit it incessantly blows, such that no one is able to put up with it nor face the pit's depths. And the name of it is 'Vith Guint' in the Briton's speech, and in Latin also 'The Blowing Wind'. It is a great wonder this wind blows from the earth."1,2
The difficulty with identifying this site centres on whether you prefer the opinion of posterity or a likely identification. Guent is almost certainly the kingdom of Gwent in South Wales3. Later Wonder-lists from England include this wonder more obviously as a cave (See Myth), but place it outside Gwent. In contrast Bradney* identifies the site as the Wynd Cliff, a very likely location, as it is near Wonders 5; 6; 8; 11 and 13.
There aren't any caves that really match this description in Gwent now, but all the caves that have winds blowing inside them are around the Llangattock Escarpment, an area where lots of caves have been carved up by quarrying. Two caves stand out: Eglwys Faen, and Pwll y Gwynt ("Pool of the Wind") directly above it*. Eglwys Faen has a strong wind just inside one entrance and a weaker breeze blowing from another4, but while the name of Pwll y Gwynt is suggestive5, there isn't actually any record of the cave having an outdraft*. The fact that these caves are the only ones associated with winds in the Gwent area6, and their general position (see Science), might suggest the cave used to be in this area, or was even some larger system including these caves, however this is very speculative. Bradney's suggestion of the Wynd Cliff seems much more concrete. Though one could argue the Wynd Cliff wind doesn't blow "from the earth", there are actually some fairly large clefts in the cliff, up through which the wind could run.
You might like to read more about the Science and Myth of cave winds, or see some visit details for Llangattock7 or the Wynd Cliff.