The Hercynian Forest was an ancient and dense forest that stretched eastward from the Rhine River across southern Germany and formed the northern boundary of that part of Europe known to writers of antiquity. The ancient sources are equivocal about how far east it extended. All agree that the Black Forest, which extended east from the Rhine valley, formed the western side of the Hercynian.
Across the Rhine to the west extended the Silva Carbonaria and the forest of the Ardennes. All these old-growth forests of antiquity represented the original post-glacial temperate broadleaf forest ecosystem of Europe.
Relict tracts of this once-continuous forest exist with many local names: the Schwarzwald ("Black Forest"), Odenwald, Spessart Rhön, Thüringerwald (Thuringian Forest), Harz, Rauhe Alb, Steigerwald, Fichtelgebirge, Erzgebirge, Riesengebirge, the Bohemian Forest, and the forested Carpathians. The Mittelgebirge seem to correspond more or less to a stretch of the Hercynian mountains.
Hercynian has a Proto-Celtic derivation, from perkuniā, later erkunia. Julius Pokorny lists Hercynian as being derived from *perkʷu- "oak" (compare quercus). He further identifies the name as Celtic. Proto-Celtic regularly loses initial *p preceding a vowel, hence Hercynia (the H- being prothetic in Latin, the Latin y signifying a borrowing from Greek). The corresponding Germanic forms have an f- by Grimm's Law: Old English firgen = "mountains" (modern German "berg" = "mountain"), Gothic faírguni = "mountain range". The assimilated *kwerkwu- would be regular in Italo-Celtic, and Pokorny associates the Celtiberian ethnonym Querquerni, found in Hispania in Galicia.
It is possible that the name of the Harz Mountains in Germany is derived from Hercynian, as Harz is a Middle High German word meaning "mountain forest." Also, the Old High German name Fergunna apparently refers to the Erzgebirge and Virgundia (cf. modern Virngrund forest) to a range between Ansbach and Ellwangen. The name of Pforzheim (Porta Hercyniae) in southwest Germany and the tiny village of Hercingen are also derived from "Hercynian".
Hercyne was the classical name (modern Libadia) of a small rapid stream in Boeotia that issued from two springs near Lebadea, modern Livadeia, and emptied into Lake Copais. It did not have any geographical association with the Hercynian Forest, so, logically, it may have been a parallel derivation from similar etymology.
The name is cited dozens of times in several classical authors, but most of the references are non-definitive, e.g., The Hercynian Forest is Pomponius Mela's silvis ac paludibus invia, "trackless forest and swamps" (Mela, De Chorographia, iii.29), as the author is assuming the reader would know where the forest is. The earliest reference is in Aristotle's (Meteorologica). He refers to the Arkýnia (or Orkýnios) mountains of Europe, but tells us only that, remarkably in his experience, rivers flow north from there.
During the time of Julius Caesar, this forest blocked the advance of the Roman legions into Germania. His few statements are the most definitive. In De Bello Gallico he says that the forest stretches along the Danube from the territory of the Helvetii (present-day Switzerland) to Dacia (present-day Romania). Its implied northern dimension is nine days' march. Its eastern dimension is indefinitely more than sixty days' march. The concept fascinated him, even the old tales of unicorns (which may have represented reindeer). Caesar's references to moose and aurochs and of elk without joints which leaned against trees to sleep in the endless forests of Germania, were probably later interpolations in his Commentaries. Caesar's name for the forest is the one most used: Hercynia Silva.
Pliny the Elder, in Natural History, places the eastern regions of the Hercynium jugum, the "Hercynian mountain chain", in Pannonia (present-day Hungary) and Dacia (present-day Romania. He also gives us some dramaticised description of its composition, in which the close proximity of the forest trees causes competitive struggle among them (inter se rixantes). He mentions its gigantic oaks. But even he—if the passage in question is not an interpolated marginal gloss—is subject to the legends of the gloomy forest. He mentions unusual birds, which have feathers that "shine like fires at night". Medieval bestiaries named these birds the Ercinee. The impenetrable nature of the Hercynian Silva hindered the last concerted Roman foray into the forest, by Drusus, during 12..9 BCE: Florus asserts that Drusus invisum atque inaccessum in id tempus Hercynium saltum (Hercynia saltus, the "Hercynian ravine-land")  patefecit. 
The isolated modern remnants of the Hercynian Forest identify its flora as a mixed one; Oscar Drude identified its Baltic elements associated with North Alpine flora, and North Atlantic species with circumpolar representatives. Similarly, Edward Gibbon noted the presence of reindeer—pseudo-Caesar's bos cervi figura—and elk—pseudo-Caesar's alces—in the forest. The wild bull which the Romans named the urus was present also, and the European bison and the now-extinct aurochs, Bos primigenius. 
In the Roman sources, the Hercynian Forest was part of ethnographic Germania. There is an indication that this circumstance was fairly recent; that is, Posidonius states that the Boii, who were allegedly Celtic, were once there (as well as in Bohemia which is named for them).
Monks sent out from Niederaltaich Abbey (founded in the eighth century) brought under cultivation for the first time great forested areas of Lower Bavaria as far as the territory of the present Czech Republic, and founded 120 settlements in the Bavarian Forest, as that stretch of the ancient forest came to be known.
The German journal Hercynia, published by the Universities and Landesbibliothek of Sachsen-Anhalt, pertains to ecology and environmental biology.
Some geographers apply the term Hercynian Forest to the complex of mountain ranges, mountain groups, and plateaus which stretch from Westphalia across Middle Germany and along the northern borders of Austria to the Carpathians.
- Aristotle, Meteorologia i.13.20; Caesar, vi.25; Tacitus, Germania 28 and 30 and Annales ii.45; Pliny, (as "Hercynius jugum", ) iv.25, as "Hercynius saltus" x.67; Livy, v.24; Ptolemy, ii.11.5; Strabo, iv.6.9., vii.1.3, 5, etc.
- Walter Woodburn Hyde noted these designations in, "The Curious Animals of the Hercynian Forest" The Classical Journal 13.4 (January 1918:231-245) p. 231. <http://www.jstor.org/stable/3287817>
- Pokorny, Indogermanisches etymologisches Wörterbuch (Indo-European Etymological Dictionary) 1959, 1059:822-23.
- Winfred Philipp Lehmann, Helen-Jo J. Hewitt, Sigmund Feist, A Gothic Etymological Dictionary, s.v. "fairguni",
- Quarqueni, a Venetic ethnicon, appears in M.S. Beeler, The Venetic Language (University of California Publications in Linguistics 4) 1949.
- Noted by Hyde 1918:232.
- John Lemprière, Lorenzo Da Ponte, John David Ogilby, Bibliotheca classica, or a dictionary of all the principal names and terms relating to the Geography, Topography, History, Literature..., (1838) s.v. "Hercyne".
- The only north-flowing river familiar to Greek and Roman geographers was the Nile.
- Caesar, Julius. "De Bello Gallico". Electronic Text Center, University of Virginia Library. pp. Book 6, Chapters 24 and 25.
- Everything of his description fits the reindeer except that the animal should have only one antler ("a media fronte inter aures unum cornu exsistit").
- The evidence for the credulous passage's not being Caesar's was first presented by H. Meusel, in Jahresberichte des philologischen Vereins zu Berlin (1910:26–29); the passage is often bracketed. "Then, as now, the local inhabitants would obviously say anything that came into their heads to a reporter in search of copy who failed to check his sources," remarks Miguelonne Toussaint-Samat (A History of Food, 2nd. ed. 2009:74) whose concern is with elk as game.
- Pliny, iv.25
- The threatening nature of the pathless woodland in Pliny is explored by Klaus Sallmann, "Reserved for Eternal Punishment: The Elder Pliny's View of Free Germania (HN. 16.1–6)" The American Journal of Philology 108.1 (Spring 1987:108–128) pp 118ff.
- Pliny xvi.2
- Compare the inaccessible Carbonarius Saltus west of the Rhine
- Florus, ii.30.27.
- Drude, Der Hercynische Florenbezirk (Leipzig) 1902 identified the plant societies in the relict forested areas.
- Gibbon, Edward. "The Decline and Fall of the Roman Empire". pp. Chapter IX, 3rd paragraph.
- Hyde 1918:231–245, pp 242ff.
- "Hercynian Forest". New International Encyclopedia. 1905.