Porth yr Ogof, otherwise known as White Horse Cave, is interesting for a number of reasons. Formed from carboniferous limestone, almost 3 km of the cave system has currently been explored.
The cave has an impressive 15 entrances, the largest of which is at the submergence of the Afon Mellte into the Porth yr Ogof cave system. At 20 m wide and 3 m high, this is the largest cave entrance in Wales. The name of the cave is derived from this entrance: the English translation is Gateway to the Cave.
If you want to know where the cave gets its alternative name, walk inside the cave as far as you can (without getting wet feet!), and then shine your torch (you did bring one, right?) on the back wall of the cave. There, you'll see the white horse, which is actually a vein of calcite running through the rock.
Porth yr Ogof holds the unenviable record of having claimed more lives than any other cave system in Britain. However, all but one of these deaths have occurred as cavers attempted to cross the notoriously dangerous resurgence pool. If you want to see the resurgence, climb back up to the car park, cross the road, and walk 200 m south along the original riverbed of the Avon Mellte to SN 9271 1220.
This is, in some ways, the easiest of the Ystradfellte series, as it's a short walk down from the Cwm Porth car park to the cave entrance, where the cache is located. Please be aware, however, that the track down is well worn, steep, uneven, and slippery. Also note that there is a £5/day car parking charge here - although there are plenty of other ways to get here, if you don't mind a little walk!
As for finding the cache, I suggest you decrypt the hints before you leave - even if you decide not to read them yet!
You will find a number printed on the inside front cover of the log book. You will need to make a note of this number in order to find the bonus cache.
Please leave the rock in the container. Hopefully it will weight it down if the river gets that high again.