## 4 Conclusions  
At a depth of 13 m, the underwater environmental conditions seem to have significantly slowed down certain oxidation reactions in the aged liquors. This is shown by a lower yellow index (b*) and lower degradation of anthocyanins. Furthermore, cellar-aged samples exhibited higher levels of 4-terpineol and ethyl 3-hydroxybutanoate, along with lower amounts of anethol, suggesting a faster aging process. Consistently, the W5C, W3C, and W1C sensors of the electronic nose, sensitive to molecules with aromatic groups such as anethol, showed a higher response for the underwater-aged liqueurs. The statistical analysis of the E-nose data pattern revealed a 96% correct classification rate, demonstrating the E-nose's ability to serve as an objective method for quality control and distinguishing between underwater-aged and cellar-aged wine-based liqueurs.  
The presence of blue-green light and sea vibrations may have contributed to an increased concentration of furanones and furans, which are known to impart toasted, caramel and strawberry aroma notes, in addition to almond notes (with benzaldehyde being one of the most abundant VOCs identified).  
Although this study investigated a longer aging period compared to recent research on underwater aging, further studies are essential to thoroughly examine the effects of underwater parameters. Sensory analysis with trained panels should be included to assess its impact on colour, aroma and taste, providing a clearer understanding of how this method affects the sensory properties. Further investigations would also be necessary to evaluate the economic feasibility of underwater aging.  
Conceptualization, A.G., S.V. and P.F.; methodology, A.G. and P.F.; formal analysis, A.B. and G.D.; investigation, A.B., G.D. and A.C.; resources, R.S., A.G. and P.F.; data curation, A.B., G.D. and A.G.; writing--original draft preparation, A.B. and G.D.; writing--review and editing, A.G., R.S., P.F. and S.V.; visualization, A.B. and G.D.; supervision, A.G. and P.F.; project administration. P.F. All authors have read and agreed to the published version of the manuscript.  
This research was carried out under the research agreement between Antica Distilleria Petrone (Mondragone, Italy) and the Department of Agricultural Sciences, University of Naples Federico II, titled \"Analisi metabolomic ed aromatica di liquori durante l'invecchiamento su fondale marino\" (Project Code: E77H23003010007).  
Not applicable.  
The data presented in this study are available on request from the corresponding authors.  
The authors declare no conflicts of interest. The authors declare that this study received funding from Antica Distilleria Petrone (Mondragone, Italy). The funder was not involved in the study design, collection, analysis, interpretation of data, the writing of this article or the decision to submit it for publication.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} E-nose & Electronic nose \\ MOS & Metal oxide semiconductor \\ SPME & Solid phase microextraction \\ GC/MS & Gas chromatography mass spectrometry \\ VOCs & Volatile organic compounds \\ LDA & Linear discriminant analysis \\ PCA & Principal component analysis \\ S & Sea underwater aged \\ C & Cellar aged \\ \end{tabular}