<p>Maars result from explosive interactions between magma, water, and bedrock. Although they are the second most common subaerial volcanic edifice, their explosive mechanisms are not always well constrained. We filled this gap focusing on the Quaternary maar de Jaude, emplaced in a marl-dominated bedrock. The maar, buried under the town of Clermont-Ferrand, offers an unusual but effective way to study the interactions between marl, water, and magma through access to several cellars. Eruptive deposits and magma-bedrock interactions were described in detail, from outcrop to micrometric scales, evidencing that 85% of the eruptive sequence consists of wet and dilute pyroclastic current deposits. Fourteen fall deposits and at least three ballistic curtain deposits were found intercalated in this 12&#xa0;m thick sequence. The proportion, size, and texture of the juvenile and non-juvenile particles helped decipher the involvement of an “impure coolant” during <i>Molten Fuel–Coolant Interaction</i> (MFCI). This impure coolant was composed of disrupted sedimentary fragments (marl fragments and felsic crystals) and water. The identification of quenched particles, up to 4&#xa0;mm in diameter, indicates that the interactive volume reached at least 33 mm³. This result is consistent with the possible involvement of sedimentary fragments smaller than this volume during MFCI. This three-component interaction process damped the explosivity compared to cases involving only water and magma. Because marls were easily disrupted by the explosions, the eruptive dynamics of the maar de Jaude were similar to those of other maars formed in unconsolidated sedimentary bedrocks.</p>