Abstract This paper reports on attempts to establish a preliminary chronological framework, based on optically stimulated luminescence (OSL) dating, for an enigmatic megalithic-like structure located underground the town of Carmona, Seville, Spain. The structure had been initially categorized as a dolmen of the Chalcolithic period. Here we applied OSL dating to superficial quartz grains extracted from the cobbles that compose the structure's walls, on the assumption that their last sunlight exposure would account for the erection of the structure. OSL dating established a maximum age for the structure that is no earlier than the 3rd Century BCE, excluding it from the major phase of megalithism of Iberia (4th–3rd millennia BCE).