The eponymous Big D has its D-shaped bath set into the end wall of a larger room, which has been paved over in modern times so it holds no clue to what was originally there. Big D is perhaps a misnomer, as the bath is of no great size, but in relation to the two other tiny baths in the adjacent area, it is bigger.
Two stucco lines on the front wall suggest a wall existed, separating an access corridor to a now-hidden staircase. The wall was removed and the small niche added later.
The Hidden Stairs
To the left of the Big D bath there is a niche and behind it a set of stairs.
There was probably a vaulted corridor leading to the staircase, completely separate from Big D. The niche was a later addition once the wall had been removed.
In the lower part of the wall between the bath and niche we can see two vertical plaster lines embedded in the stonework. This indicates that once there was a plastered wall that extended forward from here, as indicated by the red lines and shaded fill on the diagram.
In the front left hand corner of Big D itself there was once a rough opening, possibly made by treasure hunters. Paget and Jones were able to go in through here and ascend the steep stairs
The stairs are 2 feet wide and 6 feet high. At the top there are traces of another passage off to the right, however the stairs are blocked at the top. The exact nature of this staircase and where it leads should be investigated.
Armed with the appropriate page, a site inspection shows that the hole Doc and Keith climbed through has been bricked in and now forgotten.
If the stairs lead anywhere, it is likely that they end up in this room Doc called it North Tank. North tank today is only accessible from the terrace above, via an opening at the top and a tall ladder, as this picture shows.
The Mysterious Tanks
Two tanks may be accessed from an entrance directly above Big D. The entrance leads to some steps down into South Tank, while North Tank is only accessible today via an opening at the top of South Tank.
Venturing into the access corridor for the tanks, it becomes apparent from the curve of the roofline of South Tank, that there has been some adaptation. Doc Paget detected two past events, probably seismic, which had caused landfalls to bury the entrances to various features behind the hill, necessitating new access tunnels to be made, usually supported by arched tiles, as seen here.
During investigations in 2014, an opening was made and a vertical shaft revealed, with what appears to be a passage of some kind at the bottom. This has not been documented anywhere else, to my knowledge.
The vertical shaft and passage.
Nothing causes one doubt that this is a water tank, a cistern. Lined with waterproof Roman cement. In keeping with many similar tanks, steps provide access for periodic cleaning.
The floor of this tank is a rectangle measuring 4.9 metres (16 feet) by 2.7 metres (9 feet) and 4.6 metres (15 feet) high to the vaulted ceiling. The tank has its long axis aligned on 337º.
Paget estimated its base to be at the bottom step of the staircase next to Big D.
There is a small connecting opening between the two tanks at floor level, to equalise the liquid content.
The curved passage
A problem now occurs. This tank cannot always have been a tank for liquid. A treasure-hunter came into this tank at floor level.
Doc Paget and Keith Jones went in through this hole to find themselves in a 0.9 metre (3 foot) passage curving around the back of The Tholos, to a point where it likely meets up with a wall at the back of the Grotto, leading into the hill from the Greek Temple. This also means it curves directly above the main oracle entrance tunnel, the Great Antrum.
Investigating the end of the tunnel from the base of South Tank.
The lamp niche visible in the left wall matches those that we shall see in great abundance once the pages on the interior tunnels are visited.
To conclude, a corridor for humans in the base of a water tank makes no sense, unless of course the conversion to a tank was made much later.
As we saw above, access to this tank may only be made by means of a tall ladder from South Tank – there are no stairs to the floor.
This tank was clearly filled with water, through the rectangular opening shown here. The water source came from the Augusta, aka Serino, aqueduct, the main channel of which was approximately 96 km (60 miles) long and had seven main branches to Roman towns along its length including Nola, Pompeii, Acerra, Herculaneum, Atella, Pausillipon, Nisida, Puteoli, Cumae and Baiae.
A view from the platform above, into the tank.
Yet as we saw above, it is likely that this tank too was once something else, accessed at floor level from the the staircase that Doc Paget and Keith Jones recorded.
The buildings at Baia were used for many centuries, it is almost inevitable that usage would also change. The site was probably abandoned after the 472 AD Pollena eruption of Somma-Vesuvius, when the Roman Empire was in decline.