Structural control and depth clustering of extensive hydrothermal venting on the shelf of Milos Island.

High-resolution geophysical surveys, complemented by chemical, physical, and visual data collected via the ROV MARUM-SQUID, revealed a previously undocumented intermediate-depth hydrothermal vent field (30-230 m) on the shelf of Milos Island, Greece. Three primary vent areas-Aghia Kiriaki, Paleochori-Thiorychia, and Vani-are aligned with ENE-WSW and NW-SE trending active faults, which define the SE coast of Milos and the Milos Gulf-Fyriplaka graben, deepening the Milos shelf up to 230 m. All areas exhibit a distinct bimodal vent distribution, with peaks at ~ 130 m and ~ 180 m; the Paleochori-Thiorychia region additionally hosts a cluster near 210 m. Shallow vents occur on flat, sandy substrates with white microbial mats and gas emissions, whereas deeper vents feature extensive microbial mats, CO₂-degassing chimneys, and actively boiling fluids exceeding 180 °C, indicative of persistent hydrothermal activity. Although the cause of the bimodal -and, in one case, trimodal-distribution remains uncertain, our findings offer a comprehensive, high-resolution characterization of the spatial distribution, morphology, and fluid characteristics of intermediate-depth hydrothermal venting on the Milos shelf.