The Tholos

The term tholos applies to a beehive structure, uually with a hole at the top. Tholoi range in size from the size of a personal tomb to grand structures with entrance approaches, such as are seen at Mycenae. They may be tombs or they may be places to summon ghosts of the dead to the surface similar to the description of such an event in Homer’s Odyssey. 

Tholoi probably have their origin in Africa, where similar spirit houses or huts are seen in tribal villages, housing the remains and thus the ghosts of their ancestors. The idea probably came to Greece by way of Libya. The hole at the top is said to allow the soul to depart.

The Tholos here is locked and it has not been possible to enter it for many years, although I am told that at one time the padlock was simply draped around the gate and was actually open.

The Tholos Entrance at Baia
The Tholos Entrance

Although they look reasonably well centred, both the doorway and the aperture above it are not exactly true to the axis of the circular Tholos behind it, which suggests that these openings were not part of the original design and a later Roman addition.

Whatever the reason for it’s name, this is a 4.88 metre (16 foot) tall and 4.88 metre (16 foot) diameter bottle-shaped building with a 0.9 metre (3 foot) diameter vent in the roof, open to the terrace on the level above. 

The Tholos is perhaps one of the oldest buildings at Baia, but it has been lined with ‘opus reticulatum’ – ‘diamond network’ which was used from about 100 BC onwards. This is a method of stamping shaped stones, pointed at the back, into wet cement.

The original cliff was at one time at the back of the Tholos, going into the solid tufa.

The Tholos at Cuma. Drawing by G. Pellegrini, 1903.
The Tholos at Cuma. Drawing by G. Pellegrini, 1903.

There is also a Greek Tholos at Cuma nearby, about which little is known. Built with similar cyclopean blocks to those used in the Greek Temple at Baia.

I have been told that this tomb is actually quite small in size, but I have no details.

Inside the Tholos at Baia
Inside the Tholos, looking up

A Roman Sudatorium

The suspended floor, part of which still exists, shows this to have been a room heated by hot steam – a sudatorium or sweat room.

The Tholos has a passage at the back which extends for 9.75 metres (32 feet), at which point it connects to the main oracle tunnel, the Great Antrum. 

Where the tunnel meets the Great Antrum is now walled up in Roman masonry, but there is a modest Roman pipe leading through this blockage, which is conventionally how the hot steam arrived here under the floor. 

A moment’s thought and examination might lead one to question the conventional assumption.

If it is desired to preserve as much heat as possible, the last thing to do is put a heavy barrier in between the heat and its destination. Bearing in mind there are 270 metres (886 feet) of tunnel from the supposed heat source before it gets here. It makes no sense to force any remaining heat through a 15 cm (6 inch) pipe.

The passage at the back today is about half human height, so it’s is an uncomfortable crawl to the brick wall where it meets the Great Antrum.

On the right can be seen ‘suspensurae’, supports for the floor above them. Hot steam would have filtered up from below into the sweat room.

Another view, further into this tunnel. Excavation of the fill on the ground is likely reveal an original full human height, walkable, tunnel.

We can see that the tunnel is dug into solid rock here, there is no ‘cocciopesto’ rendering or lining.

Secrets of the Dead

An episode of ‘Secrets of the Dead’ called ‘Nero’s Sunken City’ premiered on March 29th, 2017. The scene here shows Dr Candace Rice, described as an expert on Roman engineering, explaining that this tunnel was built: “To access the steam vent from the natural landscape and the steam would have come into what is known as a hypocaust”.

The Heat Source?

Venturing into the Great Antrum, we can inspect the point inside the tunnel where the steam meets the Great Antrum. 

A 15 cm (6 inch) pipe is clearly visible, as is the human height Roman brick masonry all around it. The conclusion is that this wall and it’s pipe were  a later addition.

From the two pictures shown here, how likely is it that steam, having travelled 270 metres (886 feet), could come through a tiny pipe to heat a large sweat room? The steam would have had to be boiling hot at it’s source. As the source was carved by human hands and we can see door frames close by, the heat cannot have been much more at the source .

The answer is actually betrayed by a similar pipe that enters the back of the Original Entrance. That pipe has a tile drainage lip cemented over the end. It was for airflow and the condensation to flow out of, not for steam to come in.

The view from inside the Great Antrum, looking back towards the entrance.

That the pipe and brickwork has been added to block a human-sized passage can be seen very clearly from inside the Great Antrum. 

The view through the same hole to daylight shining into the Tholos allows us to see that the tunnel height here in the Great Antrum is higher than on the Tholos side. 

The height on that side has been reduced.

So where did
heat come in?

A simple examination under the front entrance to the Tholos shows exactly where. The pipe is clearly visible. Steam came in at the front, under the floor, and out at the back.
All is revealed - the Tholos Heat Source
All is revealed – the Tholos Heat Source

A feat of endurance?

If the heat is hot enough to heat a Roman bath here, how hot must it be 170 metres away?

We see signs of human activity at the heat source, so what is the maximum it could be?

A human’s tolerance to heat depends on a number of factors. High humidity lowers the ability to cope. Most humans will suffer hyperthermia after 10 minutes in extremely humid heat of 60º C (140º F) or more

For survival for any length of time, in extreme humidity, 35º C (86º F) is a maximum. We shall meet the issue of lamp niches and doorways inside the tunnels in due course and the construction of these suggest regular human use. This is not in keeping with the official suggestion of a very hot water source whose only purpose is to supply heat for a small set of Roman baths.

The bricked up wall to the Tholos
The bricked up wall to the Tholos

It’s hard to get back far enough to get a good face-on picture of the wall – after all the tunnel is only shoulder-width – but on the left wall in the foreground it is possible to see where cement and bricks have closed an original human-sized passage, which opens behind the Tholos.