The site is always open in that it is currently only visible from the outside.
When this site, located in the Phlegraean Fields, was first excavated in the 18th century, a statue of Serapis (Serapide) was discovered and the site was identified as the Temple of Serapis or serapeum. The identification was erroneous, however, as the remains are actually those of a Macellum - market building - belonging to the Roman colony of Puteoli, later Pozzuoli (see also the amphitheatre at Pozzuoli).
Having solved that conundrum, there remained another. How could it be that intact, free-standing marble columns presented a distinct band of small bore holes left by marine molluscs? This could only mean that the columns had sunk beneath the waves for centuries, remaining upright, as they did when they subsequently reemerged from the waves.
That the area is prone to seismic activity is somewhat evident, but seismic activity tends not to be subtle, tends not to leave slender marble columns intact and upright.
It eventually emerged that the land had indeed sunk and risen underneath the site, but very slowly and very gently, in seismic activity known as bradyseism, a phenomenon that proved that the Earth's crust could undergo readily measurable shifts that did not involve earthquakes. It was also observed that Pozzuoli was located pretty much at the centre of the caldera that effectively forms the Phlegraean Fields.
The market - the Macellum - was of an impressive scale, on two storeys, and surrounded by an arcade of 34 granite columns.