Ferentina is an ancient Latin Water and Fertility Goddess of the spring named after Her the Aqua Ferentina, located in the woods near Castrimoenium, the modern Marino, Italy, on the edge of Lake Albano (Latin Lacus Albanus), a large spring-fed lake rather dramatically located in the crater of an extinct volcano. She is said to have been the patron Goddess of the town called Ferentinum, a city of the Hernici (a Sabine tribe), about 45 miles south-east of Rome, off the via Latina; but it is likely the ancients confused this town with the site of Her spring and grove, which may very well have been known locally as Ferentinum after Her. The Hernician Ferentinum, now called Ferentino, is located about thirty miles from Her spring at Castrimoenium. It was originally a city of the Volscians, who were of Umbrian or Oscan origins, but it was later inhabited by the Hernici, a Sabine tribe. The town-name Ferentinum is likely derived from a reconstructed Umbrian stem *fer- meaning "to bear" or "to carry"; in Latin, which is closely related, we get the verb fero, "to bring" or "to produce", from which comes the word ferax, "fruitful" or "fertile". (These words are all related to the English word "bear", as in "to bear young", as well as the word "birth" itself.) According to that, Ferentinum would then mean "the place that is fertile"; if the town-name and Goddess-name share a root (which seems not unlikely) then Her name would mean "She Who Is Fruitful", or "She Who Bears Young". (There is another Latin word, ferentarius, meaning "lightly armed soldier"; I rather doubt it is related. In the stem form it is ferentari; stick an accent on it and you'd get Ferentári, a perfectly good Quenya word meaning "Beech Queen", probably an obscure title of Melian or something. Anyhowdyhoo—)
Hernician Ferentinum was known in ancient times as being a quiet country town, where one could escape the bustle and noise of the city. There were additionally other cities in Italy with suspiciously similar names, such as Ferentana, located in Samnium, and Ferentum, a town in Apulia. Both Apulia and Samnium were regions of ancient Italy located on the eastern (Adriatic) coast; they bordered each other, and Samnium was a neighbor of Latium, the region in which Rome, Lake Albano, Ferentinum, and Castrimoenium were located. I don't know if these cities were named for Ferentina—they may simply share a common description as places with fertile soil—but as they were neighboring territories it is not impossible that Her worship was known among the people there. (There was also a Ferentinum of Etruria, the modern Ferenti, but as it is an Etruscan name it would seem to have nothing to do with Ferentina, since Etruscan is a language all its own and not of Indo-European origin.)
Ferentina's spring was located in a sacred grove in a densely wooded valley, and a section of forest in Marino is still called the wood of Ferentina. What is assumed to be Her spring is currently found in a small park called the Parco di Colonna; it still flows quite copiously and gives rise to a brook called the Marrana del Pantano; evidentally the stream has carved out a boggy ditch, going by the name (which as far as I can tell, and I don't speak Italian, means the "steep-banked brook of the morass", unless Pantano is simply a name). The area around this spring has been inhabited since pre-Roman times, and there is evidence that the local people established trade with the Etruscans to the north. At this grove Ferentina also had a shrine, which was famed as the meeting place of the Latin League, a confederation of cities of Latium in the early days of Rome. The Latin League met there regularly until the mid-4th century BCE; about that time the cities of Latium, which had long been at odds with Rome, were defeated by that city and absorbed, amoeba-like, into its ever-expanding territory. There were traditionally 30 member-cities of the Latin League, and one imagines the shrine and/or meeting place must have been of a good size; in modern times there were said to have been Roman ruins near Her fountain, though there is little left today.
Tarquin the Proud, traditionally the last King of Rome before the Republic (and son-in-law of Servius Tullius, the guy who built all those temples to Fortuna) was said to have framed the Latin leader Turnus Herdonius, who had spoken against him; while the Latins were all assembled at the shrine to Ferentina, Tarquin made it seem that Turnus had plotted against the other Latins as well as himself by smuggling a whole bunch of weapons into Turnus' home, making it appear that Turnus was getting ready for a murderous spree. The Latins, who didn't much like Turnus anyway, were all too ready to believe Tarquin, and upon finding the weapons were so angry that they would not even hear his defense, and had him immediately executed by drowning him in Ferentina's spring.
In another Roman tradition of the early days, set down by Plutarch (who lived in the 1st-2nd century CE), Rome was said to have been visited by a plague because Romulus, the very first King, had not sought justice in the murder of Titus Tatius, the former King of the Sabines, who for a time had ruled Rome jointly with Romulus. He was finally persuaded to perform the necessary rites of purification, and the plague abated. Plutarch then says that these rites were still being performed at the grove of Ferentina in his time.
Ferentina had another cult center at the city of Aricia, located sort of between Lake Albano and Lake Nemi (aka Nemorensis), another, smaller, lake-in-a-volcanic-crater only a few miles from Lake Albano. The famous grove of Diana Nemorensis was located there, in which grew a "golden bough" in a sacred oak, probably mistletoe, and which featured in the ritual by which Her priests were replaced. The name Aricia may be linked to the Latin verb arare "to plow"; if so, its name would mean "place of arable land", which is not dissimilar to the meaning of Ferentinum ("place that is fertile"); perhaps Ferentina as a Goddess of fruitfulness has something to do with the name; perhaps it's just that the soil around volcanoes is especially fertile.
Though little is said or known about Ferentina Herself, Her attributes can be pieced together from the stories told of Her holy places. She is, of course, a Goddess of Water and Springs, specifically the Nymph of a spring central to the Latin peoples; and springs are traditionally symbolic of origins, birth, and connections with the Underworld, as they emerge directly from underground. She is a Goddess of the Earth and the powers of fertility, fruitfulness and birth; and She is especially associated with land good for farming, and by extension the civilizing powers of agriculture, much like Ceres. She watches over Her people, the Latins, and protects Her patron cities. But there are also hints of a darker side to Her, in the association of both springs and the dark earth with the Land of the Dead; and two of the tales mentioning Her grove involve murders. In Turnus's murder the Latins make doubly sure he is gone by sending him straight to the Underworld not only through his death, but by the medium of a spring, considered a gate to the Underworld as well as a proper place to send sacrifices to the Gods; and the traditional purificatory rituals performed at Her grove are also linked to the murder of a leader or King. One could, I suppose, also infer a veiled reference to the idea, common in myth, that the King, like the year, dies or is killed annually, to be replaced with a younger, stronger, more virile version; if that holds (and I suspect it is the proximity of the grove of Diana Nemorensis to Ferentina's that put it in my head) then perhaps Ferentina is a Goddess of the process by which the end and the beginning of a cycle are smoothly joined, in the recognition, that though necessary or even proper to the general well-being, the King's death is a disruption still, and that right order must be restored through the proper rituals.