The calanques are the astonishing rough diamonds of the Mediterranean coast, an essential highlight of any tourist trip to Southern Provence.

A spectacular series of looming white limestone rocks scored through with deep valleys, they're the backdrop to a unique ecosystem, colourful fishing villages, peaceful creeks with intense, clear turquoise water and a wide range of sporting activities.

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The calanques (pronounced "kalonk") extend for almost 20 km (12.4 miles) in length and 4 km (2.4 miles) inland south-east along the coast between Marseille and Cassis, snuggled in the folds of a mountain range of which the highest peaks are Marseilleveyre (432 metres / 1400 feet) and Mont Puget (565 metres/ 1850 feet).

On a large-scale map (click on the map to enlarge the image), some two dozen calanques of different sizes can be identified along this strip.

There are also smaller calanques to the west of Marseille along the Blue Coast, between the city and Carry le Rouet.

This article is a general introduction. Click here to read a guide to the best calanques south of Marseille, here for a guide to the calanques west of Marseille and here for a guide to the calanques near Cassis.

THE ECOLOGY OF THE CALANQUES

The calanques have an extreme microclimate that produces a range of rare vegetation. In winter, the temperatures in the sheltered valleys can be sigificantly warmer than in the surrounding area.

On the other hand the limestone is highly porous and it rarely rains: the main source of moisture rises from the salty evaporation of the sea in the blazing hot sun.

And the soil is almost non-existent. Instead, the cliffs are criss-crossed with cracks into which a distinctive array of tough, determined plants wedge their roots.

Though the rocks look barren, they conceal approximately 900 plant species, 15 of which are protected. Depending on the sheerness of the slope and the direction it faces, you may find Mediterranean oak, Aleppo pine, wild olive trees, viburnum, myrtle, mastic, criste-marine (a form of samphire), sea lavender, an array of herbs including rosemary, thyme and fennel and Marseille Astragalus, a thorny plant unkindly known as mother-in-law's cushion.

The calanques are also home to rabbits, foxes, wild boars and many reptiles including the ocellated lizard - the largest lizard in Europe - and Montpellier snake.

Birds are less populous, but you will see plenty of seagulls and, if you're lucky, a peregrine falcon, a great horned owl or a Bonelli's eagle (pictured). This French-language website provides a comprehensive guide to the flora and fauna of the calanques.

A THUMBNAIL HISTORY

What's in a name? Depending on which source you consult, calanque comes from a Latin, or Corsican, word, calanca, meaning a rocky inlet, or else the provençal cala, meaning a steep slope).

Though they're often called Mediterranean fjords, the calanques were not caused by glaciers like their Scandinavian cousins.

They were originally created by rivers coursing through a faultline in the rocky mountain chain. When the sea-level rose, the canyon was partly flooded, leaving a precipitous, narrow creek.

The calanques extend into the sea as long underwater valleys and the water there is significantly colder than elsewhere along the coast.

For centuries the limestone rock has been quarried and used all around the world, from New York's Statue of Liberty, for which it supplies the base, to the Suez canal and the quays of Alexandria. Closer to home, the rock was used for the lighthouses in Cassis and Marseille. One of these old quarries can be seen today in the calanque of Port Miou.

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It's the great traditional dream of Marseille city-dwellers to buy their own cabanon, or weekend cottage, in one of the calanques.

Originally implying a fisherman's cottage, today a cabanon can mean accommodation of all kinds, from little shacks to modern gites or luxury villas (pictured: a cabanon in the calanque of Sormiou).

Locals visit their cabanons regularly throughout the summer (and sometimes in winter too), using them as the perfect base for fishing, hunting or swimming, a sun- and rosé-drenched lunch and a game of pétanque or cards.

Some calanques such as Morgiou or (among the calanques west of Marseille) Niolon and Méjean are lively working villages with a year-round population.

Today this fragile environment is at risk from pollution, over-fishing and - because of the arid climate combined with high winds - forest fires. A massive inferno engulfed 2900 hectares (7000 acres) in August 1990 and, despite increased precautions, it remains an ever-present threat.

For this reason the calanques between Marseille and La Ciotat (with the partial exception of Port Miou, near Cassis) as well as the offshore islands of Frioul and Riou, were declared a National Park in April 2012. It is the seventh such one in mainland France, and only one of three in the world that's right on the edge of a major city, in this case Marseille.

Restrictions are placed on certain activities such as fishing, hunting and driving in the National Park. Click here to read the full rules and regulations governing visits (in French only).

HOW TO VISIT THE CALANQUES

The extraordinary topography of the calanques makes it difficult to get to them, except by boat. And because of the severe fire risk in summer, access on foot and by car is restricted by law between 1 June and 30 September.

During this period there are three risk levels: orange (access authorised), red (access authorised between 6am and 11am) and black (access banned).

The level varies according to weather conditions and is set daily at 6pm for the following day. It's available on the official helpline, tel (+33) 8 11 20 13 13, in English as well as French and published (in French only) on the Bouches du Rhône regional website.

On "black" risk days, visiting the calanques by boat or train are the only options (and even the boats may not operate on days of very high wind).

By boat: Excursions of varying lengths run out of Marseille, Cassis and La Ciotat along the eastern calanques, and from Carry le Rouet along the western calanques. This is the best way to take them in relatively quickly.

It's not so good if you want to get close to nature or to experience a whiff of calanque culture. You see them from the sea accompanied by a booming loudspeaker commentary; for safety reasons, passengers are not allowed to get off the boat at the calanques. On the other hand, a boat trip offers an excellent overview of the territory prior to planning a longer, more intimate visit.

From Marseille: Croisières Marseille Calanques, Icard Maritime or Blue Attractions

From Cassis: Calanques-Cassis

From La Ciotat: Les Amis des Calanques

From Carry le Rouet: L'Albatros Côte Bleue

By train: The south-eastern calanques cannot be reached by rail. However a petit train touristique (little tourist train) and a shuttle bus run from the centre of Cassis to the nearest calanque, Port Miou. Click here for a full guide to how to visit the calanques near Cassis.

The western calanques are served by the Blue Coast train line which runs from Marseille to Miramas. There are about a dozen trains a day in each direction and it takes between 20 and 35 minutes to reach the calanques.

The train ride in itself is very beautiful for the "calanque" part of the route, between the stations of L'Estaque and Carry le Rouet, pictured, from where you can take a boat trip back along the coast to see the calanques from the other side.

These calanques are less dramatic than their eastern counterparts. However, it is a good way to see the smaller ones if you are short of time, have restricted mobility, are reluctant to pay for the (rather expensive) boat trips and/or just want to go out to a calanque for a nice lunch.

By car: You can drive to certain calanques outside the high summer. The main ones and how to get to them will be detailed in our calanque guide. However, be aware that, on a fine spring or autumn day, especially at weekends, parking will be in extremely short supply.

In summer it is sometimes possible to arrange a pass to drive down to certain calanques by telephoning one of the restaurants, such as the Nautic Bar in Morgiou or Le Lunch and Le Château in Sourmiou, as long as you are intending to eat there. You must book a table well ahead, and it's likely that you will need to be able to speak French to make the phone call.

Car-parks in all the calanques are notorious for break-ins during the tourist season. Do not leave valuables in your vehicle and try to retrieve it before nightfall.

On foot: A part of the GR51 - GR98, the Grande Randonnée (long-distance footpath) is a 28 km (17.4 mile) trail that traverses the calanques from Marseille to Cassis.

Many parts of this route are steep and challenging and even an experienced hiker would be hard-pressed to cover it in a single day.

You are strongly advised to arm yourself with a good large-scale map. Find a large-scale IGN map of hiking trails through the Marseille-Cassis calanques on Amazon

If you read French, consider investing in Les Calanques à pied, one in the series of definitive topoguides (illustrated guides) produced by the Fédération française de la randonnée pedestre, the national rambling federation. The one for the calanques offers 28 hikes of varying lengths and levels of difficulty.

Camping out in the calanques overnight is prohibited by law. It's therefore best to walk this trail in separate, smaller stages.

Even on days when access is authorised, the heat - amplified by the sun and reflected by the sea and the white rocks - will make hiking disagreeable in the middle of the day. Plenty of water and protective headgear are essential, as well as the usual equipment.

Parts of this trail skirt vertiginous drops and so it's dangerous on days of the Mistral (Provence's fierce, gusting wind). You should check the weather report before setting out.

Several calanques can be visited individually on shorter excursions. For example, from the Luminy University complex you can walk to Sugiton; from Callelongue to Marseilleveyre, Queyron and Podestat; from the La Cayolle car-park to Sormiou and L'Escru; from the car-park near the Baumettes prison to Morgiou and Sormiou (pictured left); and from the Port Miou car-park in Cassis to Port Miou calanque, Port Pin and En Vau. See our articles on the best individual calanques for more details.

There are several other options for sportier visitors. One is is to run in the popular "Marseille-Cassis - 20 km" race. It takes you along the road (which is closed to traffic that day) along the top of the calanques.

It's on the last weekend in October and there is also an organised hike along a similar route on the previous day. Click here to read about the Marseille-Cassis race and "L'Autre Marseille-Cassis" organised ramble.

Another option is to go on an excursion into the calanques near Cassis via the Bureau des Guides des calanques (the Office of Guides to the Calanques). They are available for groups of a minimum of four and a maximum of 12 persons. As well as simple guided walks, canyoning, abseiling (rappelling), mountain biking and rock climbing are all on offer. Some excursions are available in English.

The Bureau des Guides des calanques is at the Tourist Office on the quai des Moulins, 13260 Cassis. Tel: (+33) 6 85 55 04 47. Website for the Bureau des guides des calanques

It is possible to arrange conventional guided walks through either the Marseille Tourist Office or the Cassis Tourist Office.

The Cassis Tourist Office also has a list of places where you can hire a kayak. Kayaking to the calanques and in Provence will be covered in full in a future article.