Traditional domed houses and narrow alleys built along the ridge of the caldera.

ABOVE ME, THE 200M cliffs of the Santorini caldera provide an impressive background. Even more inspiring is the knowledge that these cliffs continue almost 300m below.
Captain Dimitris and dive guide Manthos anchor the boat carefully at the end of a submerged ridge at Adiavati. The Santorini caldera is not the result of a single eruption; over half a million years and more, successive eruptions have superimposed one caldera on another, leaving a submerged outcrop of an old rim to interrupt the perfection of the wall.
We descend to a plateau at 8m, interrupted by a few pillars of rock that come close to the surface.
Dimitris confirms that the shiny stainless-steel anchor is wedged in a crevice, though he also leaves a friend on board to keep watch. Its a fitting anchor for a luxury motor yacht; stainless-steel chain sparkles in the sun all the way up to the bow.
I have to make an effort to think about depth as my judgment adjusts. The submerged wall descends way beyond the limit of typically perfect Mediterranean visibility.
Colour is provided by frequent patches of encrusting, boring and branching sponges in yellow, orange and brown. Fish are present, but not plentiful. Grey-brown chromis cloud about the shallower outcrops. Small rainbow and peacock wrasse are forever busy and curious, darting about constantly. Two-banded bream prefer the shallower parts of the wall, but only in twos and threes.
At 35m I can see a boulder-covered shelf below me with grouper, at least 10 and perhaps as many as 20. I descend towards it, and the grouper shy away. Grouper are cautious fish, and there is no point trying to approach closer.
As with any dive with big scenery and visibility to match, I have to make a conscious effort to look more closely
at the wall. It doesnt take long to find a good variety of nudibranchs, gobies and other macro life.
We head across the ancient entrance of the caldera to Aspronisi island for lunch and a second dive.
In the time of Atlantis, this gap would have been considerably smaller, and the only entrance. Nowadays, it is one of three bigger breaks in the rim.
Aspronisi is a small and uninhabited island surrounded by crumbling cliffs that rise 30m from a thin strip of beach. Like all the caldera dive sites, it goes steep and deep from just off the beach.
On the wall, I admire a preponderance of orange branching sponges, fed by a barely perceptible current. Banks of primary green break the wall in truncated valleys of sea grass. The 26C shallow water gives way to 20 at 25m. I could feel the thermocline at Adiavati. Here I can also see and even photograph it, as tiny dust particles trapped in the brief interface twinkle in the sunlight.
The dive boat Psarou III is a beautiful motor yacht, all sparkling white and chrome, with teak decks on which you cant wear high heels (not that I would). The sort of boat that looks incomplete without a swimsuit model on the bow.
In a calm sea it cruises at 30 knots. With a strong wind from the north-west, life is more comfortable at 15 knots. Captain Dimitris runs the luxury end of the charter business. For divers like me who are not celebrities, guide Manthos also operates a couple of Humber RIBs.
I alternate dive boats between Psarou III and the RIBs, using whichever is most convenient for a particular dive.
If I was an A-lister like those the captain caters for, I would have someone from my entourage see to it that I didnt slum it in anything less than a 36m luxury yacht.
I dive from Psarou III for another couple of dives inside the caldera rim - a submerged arch at Firostefani, and another steep and deep wall at Skaros, a headland to the south of Oia.
Like any arch, Firostefani provides the rich and colourful sessile life that comes with shelter from sunlight and the surge of the waves.
For anyone who has ever received a postcard from Santorini, Oia (pronounced ee-aa) is the town of narrow alleys and domed white buildings clinging to the ridge that could have been made for postcard pictures. Tourists flock here to watch the sunset.
Between dives, I pick at a bowl of local grapes Dimitris provides for just this purpose. They are so sweet and juicy that I buy a bunch from a stall on the seafront in Kamari, just along from the Dolphins Apartments where I am staying, and where Manthos runs the Volcano Dive Centre.
I pay 4 euros for a carrier-bagful, then the man balances another bunch on top. They make breakfast, lunch and an early-evening snack for two days.

THE DIVING IS NOT JUST IN THE CALDERA. Some canyons, caves, tunnels and more arches show on the waterline at Indianos, just below the Akrotiri lighthouse that guards the southern end of the old entrance to the caldera.
This is one of the dives on which a RIB is the better boat; the shallow water and big rocks just wouldnt be safe for Psarou III.
Some of the caves are easy, straight in and out; others have branches and second entrances. One gets so dark and narrow with the possibility of branches that, without a line, I turn round rather than push to the end.
I enjoy a day exploring the sort of shallow site with which any diver can have real fun.
The largest cave is a tunnel 100m long with a big entrance, a narrow exit and a T off to one side that goes back to a chamber with air. Its the favoured residence of a monk seal, though he isnt at home when I look.

BEFORE THIS TRIP, my excitement had grown substantially when the cruise ship Sea Diamond struck the rocks and it looked as if Santorini was going to have its own Zenobia.
When you consider what the Zenobia has done for diving tourism on Cyprus, it is easy to imagine that Dimiotris, Manthos and other divers on Santorini were excited about the prospect of having such a significant wreck.
It took 15 hours to sink. Having towed the ship off the rocks to get the passengers off, you would have thought they could have towed it somewhere shallow enough to make
a decent dive site. Looking at the chart, I can see enough places that are flat enough, shallow enough and away from anywhere that would obstruct shipping.
Instead, the wreck of the Sea Diamond was allowed to slide down the wall. The bow is now in 80m and the stern straight down from that to beyond 200m, and it could slide deeper.
While I feel sorry for the two passengers who were lost, its hard for me to think of the wreck in any context other than as a wasted opportunity.
The operator Louis Cruise Lines has since been fined 1.17 million euros, and the captain and five other officers have been charged with negligence.
At the opposite end of the scale, a yellow submarine tour company sank a tugboat in 35m as an attraction at Palea Kameni. The submarine business has moved on, but the tug is still there for me to enjoy.
Palea Kameni (Old Burned Island) is the result of lava flows in the centre of the caldera since 1600 BC, building up from the seabed to break the surface in 46AD.
Next to it, Nea Kameni (Young Burned Island) formed slowly between 1707 and 1711. It is still a centre of volcanic activity, with the most recent eruption in 1950.
Eruptions are roughly 150 years apart, so the next is due in 2100. Big eruptions are about 15,000 years apart.
Volcanic activity isnt limited to the Santorini caldera. A string of submarine volcanoes follows the faultline off to the north-east. The biggest is Kolombos, five miles out from Santorini, which emerged briefly from the sea in 1650.
Fifty people on Santorini were killed by the resulting tsunami and cloud of sulphur dioxide.
The cinder cone that broke the surface has now eroded to firmer rocks that peak 20m down. Outside, the sea drops to 230m. Inside, the Kolombos caldera drops to 505m. If someone had asked me to pick an exciting dive site from the navigation chart, Kolombos is where I would have poked my finger.

DIMITRIS AND MANTHOS HAVE already poked their fingers, and use my visit as an excuse to take a look. As far as Manthos knows, its a virgin dive site.
In a rough sea, the big hull of Psarou III copes as well as any UK charter boat. GPS plotter and echo-sounder make easy work of finding the shallowest point of Kolombos at 20m.
Manthos and I jump in. Dimitris plays it safe and stays on board. An extra pair of eyes on the surface is the prudent option in the waves.
Finding the shallow point of Kolombos is easy, but its a good 15 minutes into the dive before we find the inside slope of the caldera. Its a rounded ridge slowly getting steeper, but remaining a 30 slope at 35m. The colour of the clouds of small fish, fine algae and vibrant sponges sparkles.
If I was really A-list, I would have a personal decompressor in my entourage. That way, I could go straight to the surface as the decompressor jumped in to complete my stops. More practically,
I could charter the boat for the next month and really explore Kolombos.

The many legends about the ancient land of Atlantis all stem from repeated embellishment of a few paragraphs written by the Athenian philosopher Plato around 400 BC.
Plato wrote about Atlantis disappearing either 900 years or 9000 years before the time of Solon (depending on which translation you believe).
Solon was an Athenian statesman and poet who lived a couple of centuries before Plato. Plato had heard the story from his grandad, who heard it from a friend, who was told the tale by Solon. Solon heard it from some Egyptian priests.
So we have a story well embellished before and after Plato, and perhaps embellished by him for political reasons. Like any other diving story, it grows with the telling and the alcohol consumed.
We cant even depend on the name Atlantis, which would not have been used by Egyptians. Plato or an earlier Athenian introduced it after the mythological Titan Atlas.
What we are left with is a land that may have disappeared beneath the sea amid earthquakes, towering black clouds, thunder and lightning, and the demise of a prosperous civilisation, around 1500 BC.
It may have been beyond the Pillars of Heracles (Hercules to the Romans), but that depends on knowing what the Pillars of Heracles are.
Give or take a few years, the timing of Platos Atlantis just happens to be close to the
Thera eruption of about 1600 BC, an explosive event with a bang five times the size of Krakatoa.
Santorini is the modern name for the group of islands on the caldera rim of this volcano (seen right in this satellite shot).
Thera, the name of the main island on the rim, is often used by geologists and ancient historians when referring to the single island that existed before eruption broke the rim.
Before 1600 BC, the rim was wider, flatter and almost complete. The one small entrance in the surrounding cliffs at the south-west connected the flooded caldera to the open sea. About half of the current area of the caldera was flat fertile land. It may have been connected to the rim, or been an island inside a natural moat.
Whatever the topography, the land and seabed of the caldera was blocking the top of a volcano. Over tens of thousands of years the pressure below built up until it erupted through, spewing out 15 cubic miles of rock and ash.
Santorini changed shape. The caldera rim was broken into three main islands.
New layers of rock more than 30m deep were deposited on the remaining land.
Thick layers of ash from the eruption have been found on surrounding islands and as far away as Turkey.

The Thera eruption raised a tsunami that swept the north coast of Crete 70 miles away. Ash in the atmosphere caused a dip in the climate, and harvests failed across the Eastern Mediterranean. The tsunami and famine are thought to have triggered the collapse of the Minoan civilisation centred on Crete.
Archaeological excavations on Santorini have uncovered the remains of a town from this period, more than 30m below the compacted ash, with three-storey houses with indoor plumbing and running water. The people were either Minoan or part of an equally sophisticated society.
No bodies have been found. Perhaps the volcano grumbled long enough before the eruption that the population had time to flee.
Further afield, some speculate a connection between the Thera eruption and the plagues, famines and unusual rains of the Biblical exodus, which occurred at about the same time.
While we are speculating, consider the mushroom cloud from the explosion. Could an ancient Egyptian have described it as like a tall elemental giant rising from the sea, hunched over and carrying the sky on his shoulders - perhaps the Titan Atlas
Could the small gap providing access for ships through the cliffs of the caldera wall be the original Pillars of Heracles, beyond which lay Atlantis
Orange branching sponges on the wall.
The Indianos Caves in the cliffs below Pharos.
Psarou III in the bay beneath Oia.
Loading the truck to ship dive kit to the harbour.
The Volcano divers ready to leave Vlychada harbour.
Diver on the wheelhouse of the tugboat wreck at Pala Kameni
Ladder between the decks at the stern of the wreck.
Golden goby.
Manthos explores the Kolombos site.
Nudibranch on a sponge.
Barrel sponge.
GETTING THERE: First Choice packages and flight-only charters from Gatwick. Flights are via Athens, or high street shops. Divers baggage allowance is easier to arrange through the shops.
DIVING:Dive Safari / Psarou III, Volcano Diving Centre, accommodation Dolphins Apartments,
WHEN TO GO:The dive centres shut down for the winter. The season runs roughly April to October.
PRICES: Flights cost from £350 return. A private charter on Psarou III starts from 1000 euros. Diving with Volcano Diving Centre costs 80 euros a day. Dolphins Apartments cost from 50 euros a night for an apartment sleeping two.