Abstract In this paper we investigate lexical materials in Latin and Romance that appear to stem from ancient substrata in the Mediterranean. We focus on plant names and landscape related vocabulary, e.g., words for ‘berries’, ‘shrubbery, herbs and trees’, and words for ‘rocky’ and ‘swampy’ terrains, as both of these semantic fields are parts of the lexicon considered to be of very archaic origin. After investigating evidence for potential cognates in languages in the Mediterranean and exploring the geolinguistic distribution of the investigated words, we try to form conclusions about the involved strata and their chronological organization. We identify two important strata: an older Eurafrican layer (Hubschmid 1960) and a widely attested Euskaro-Caucasian layer associated with the arrival of Neolithic farming (see the discussion in Bengtson (2017a/2022, 2017b) and Bengtson/Leschber (2019, 2021, 2022)). The so-called ‘Mediterranean Thesaurus’ (in memory of Johannes Hubschmid) aims to establish a compilation of lexical data featuring words with unclear or highly disputable etymologies in the languages around the Mediterranean Sea. As a first step, we are collating words in an open-ended table and their geolinguistic/dialect-geographical distribution across the ranges of the Mediterranean languages, some of which are geolinguistically remote areas. We trace two main questions: (a) whether there are characteristic semantic fields that can be more often identified (such as plant names and topography); (b) whether we can highlight typical phonetic features and sound clusters in these words. Since the period of the Swiss and Italian etymological pioneers of the 20th century (Johannes Hubschmid, Giovanni Alessio, Carlo Battisti, Vittorio Bertoldi, Jakob Jud, Laura Lombardo, Alfredo Trombetti and many others), there has been great scientific progress in human genetics and archaeology that is increasingly revealing the prehistory of the Mediterranean. We recognize the work of these etymologists who – under much more demanding circumstances – offered suggestions about the origin of etymologically difficult words. Several hypotheses have been revisited in order to include or to reject the contribution of a specific substrate to the linguistic substrata of the macro-area. Our aim is to explore the prehistory of the Mediterranean area to understand the ethnogenesis of its peoples and their rootedness in millennia past, and to find evidence for a tentative stratification of substratum languages.