Preventive conservation and restoration of mural paintings in hypogean environments is a significant challenge. These types of settings are particularly difficult to manage due to their peculiar morphological and microclimatic features. Since its foundation in 1939, the Central Institute for Restoration (Istituto Centrale per il Restauro—ICR) within the Italian Ministry of Culture has been engaged in the prevention and safety of these unique cultural heritage assets. Starting from a holistic analysis of vulnerability and hazards specific to hypogean environments, this study examines the main risks and corresponding conservation strategies. Particular attention is given to the impact of residual risk on restoration decision-making, with the aim of defining logistical and operational requirements to carry out sustainable and enduring interventions in these complex settings. The compatibility and durability of restoration materials under hypogean conditions are also examined. Furthermore, thanks to funding provided by the PNRR CHANGES project, a few research directions are proposed to address unresolved issues through the investigation and assessment of innovative products and methodologies. This review aims to lay the foundation for the development of guidelines incorporating protocols for the conservation and restoration of mural paintings in hypogean contexts, with particular attention to the needs and constraints imposed by these specific environments.

Consolidation treatments are essential for the conservation of wall paintings affected by decohesion and disintegration phenomena. In hypogean environments, high relative humidity, limited ventilation and elevated biological risk impose particularly stringent performance requirements. Under these conditions, consolidants must ensure chemical compatibility, effective distribution within water-saturated substrates, long-term stability at high relative humidity and low toxicological impact. Calcium-based nanomaterials, especially nanolime dispersions, are widely employed as reference consolidants. However, their performance is strongly influenced by the dispersing medium, environmental conditions and substrate characteristics. This study addresses the lack of comparative assessments of currently available calcium-based consolidants by testing four products—two alcohol-based nanolimes (Nanorestore Plus^® and CaLoSil^®), one aqueous nanolime dispersion (Nanolaq^®) and a laboratory-formulated aqueous nanocalcite—applied to painted mock-ups. For the first time, the effectiveness of these treatments was investigated under both high relative humidity and water-saturated conditions through a multi-analytical approach. Colorimetric variations, water vapour permeability, water absorption and consolidant distribution within the pictorial layers were evaluated. The results provide a comparative assessment of consolidant performance as a function of the hygrometric regime of the substrate, with differentiated responses under high-humidity conditions and attenuated differences under water-saturated conditions. Overall, the effectiveness of the consolidant appeared to depend significantly on the combined influence of the dispersing medium, the imbibition state of the substrate and its chemical nature, highlighting the need for selection criteria and evaluation protocols based on simulating realistic conservation conditions.