Aquan languages
The Aquan languages are a hypothetical language family proposed by Jörg Rhiemeier. The languages are all extinct and unattested, leaving only traces in other languages, mainly in form of the Old European hydronymy. The name "Aquan" is derived from Latin aqua 'water', a word that is limited to the westernmost branches of Indo-European (Italic, Celtic and Germanic) and also occurs in the Old European hydronymy, and may have been the Aquan common term for a watercourse.
According to Rhiemeier, the Aquan languages were spoken in Neolithic Central and Western Europe and related to the Indo-European family (see Europic). Proto-Aquan may have been spoken around 5000 BC by the people whose archaeological remains are known as the Linear Pottery culture. From there, the languages were carried north by the Funnel Beaker and west by the Bell Beaker peoples. The last holdout of Aquan languages may have been in the British Isles, where these languages may have been the substratum responsible for the "un-Indo-European" appearance of the Insular Celtic languages.
The Aquan languages seem to preserve some archaic features of an ancestor of Proto-Indo-European, such as a three-vowel system (*/a i u/) without ablaut, and a more agglutinating morphology. Substratal evidence from the Insular Celtic languages may indicate that the insular branch, at least, may have been head-initial and active/stative, with a tendency to phonologically run together syntactically closely associated words (such as noun and adjective within a noun phrase).
Aquan conlangs
Jörg Rhiemeier explores the lost world of the Aquan languages speculatively in his Hesperic conlang family within the framework of the League of Lost Languages.