A
cryptoporticus (stone domed structure) that - considering the remains found on location, the architectural characteristics and the techniques and materials used - served to support the
baths. Discoveries include architectural elements, moulded and painted plaster, wooden and glass trays, ceramic tiles, in addition to common and fine ware and
amphora, from the High Imperial Roman Period and Late Antiquity. Archaeological research concluded that water infiltrated the subterranean galleries shortly after its construction in the 1
st century AD, through a still visible crack.
The structure was discovered inundated by water, during the pombaline reconstruction following the earthquake of 1755. During the 18
th and 19
th centuries, surveys and descriptions led to different historic interpretations, but only in the late 20
th century did archaeological interventions begin, in particular with the Project to Study and Value the Roman
Cryptoporticus of Lisbon (a municipal plurennial archaeological research program).