Despite the name, this is not a palace. It's an extravagant, extraordinary, excessive monument to the glory of water and its critical importance to Marseille ever since the city was founded by the Greeks as a trading port in 600 BC.
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With an average of only two days of rain a month in the midsummer, drought has been a perennial problem for the city (it is, of course, the central theme of Marcel Pagnol's famous stories, Jean de Florette and Manon des Sources).
A series of cholera epidemics finally prompted the authorities in 1834 to commission a canal linking the city to the river Durance.
Besides providing a water source, it would connect Marseille to the national waterway network and open up new trading routes, a matter of some urgency following France's invasion of Algiers in 1830 and its other colonial activities in the Maghreb.
80 km / 50 miles long, including 17 km / 10.5 miles of underground tunnels, the canal took 15 years to build and required 18 aqueducts.
It was opened in 1849, and remained Marseille's principal source of water until 1970, clearing the way for the city's economic growth and prosperity. A suitably imposing monument was thought appropriate to celebrate its completion, and thus the Palais Longchamp was conceived.
Designed by Henri-Jacques Espérandieu, the architect of Notre Dame de la Garde, the result, inaugurated in 1869, is a stupendously large fountain/waterfall/water tower of a positively Parisian opulence, embellished with colonnades, staircases, pavilions and a triumphal arch and lavishly decorated with sculptures of nymphs, stalactites, lions, tigers, dolphins and Camargue bulls.
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Located in a quiet residential suburb east of Saint Charles Station, the Palais is surrounded by spacious grounds traversed by the final part of the canal in an aqueduct. Apart from the green space in front of the monument, there are three big, interconnected parks behind the Palais Longchamp.
One of these was a zoo in the 19th century, and you can still see many of its picturesque buildings in fantastic styles, including pavilions for giraffes and elephants, cages ornamented with Turkish tiles, and seal dens decorated with rocaille, or rock-work. Pictured: this oriental giraffe house has now been converted into a little children's theatre.
In 2013 this part of the Palais Longchamp's grounds was repopulated with whimsical, brightly-coloured fibreglass animals to form a "Funny Zoo".
With its ancient trees, fine views across Marseille (Notre Dame de la Garde can be glimpsed in the distance above the bull's horn, above right), plentiful places to picnic and several children's playgrounds, the Palais Longchamp is an attractive, restful excursion on a summer's day to escape the heat of a city not famed for its squares, parks and leafy spaces.
This is one of its "green lungs", alongside the Parc Borély and (somewhat further out of town) the Campagne Pastré. On a warm weekend it's packed with locals out for a picnic or a stroll.
In July, the huge park at the Palais Longchamp is the main venue for Marseille's prestigious annual Five Continents Jazz Festival, known as FJ5C to cognoscenti. The 2013 Five Continents Jazz Festival featured Diana Krall, George Benson, Gilberto Gil, Archie Shepp, Chucho Valdés, Hiromi and Wayne Shorter, among others.
Tickets to the top acts at the Palais Longchamp jazz festival can be pricey: as much as 40 €uros. Visit the park instead in the afternoon, when the artists can be heard rehearsing - and admission is free.
Also of interest are the museums occupying the Palais's two wings, the Museum of Natural History and the Museum of Fine Arts. Recently refurbished, the latter reopened to the public in June 2013.
In the park behind the Palais are a small observatory, with a planetarium (pictured), an exhibition, Galactic Journey, and Foucault's enormous historic telescope - once the largest in the world - dating from 1864.
Across the road is the Grobet-Labadié Museum, which recreates the intimate and comfortable atmosphere of a wealthy 19th merchant's house, with tapestries, wood-carvings, musical instruments, chinaware and paintings; even a sedan chair.
Where: Palais Longchamp, Boulevard du Jardin Zoologique, 13004 Marseille.
How to get there: Metro (line 1, direction La Rose; stop Cinq Avenues Longchamp), tram line 2 or bus 81.