FOURTEEN years ago Spanish photographer Joan Fontcuberta landed a bizarre assignment in Provence, southern France.

He was invited to photograph the remains of an unknown species discovered on a beach and in surrounding caves. On arrival, he was astonished to find the fossilised remains of "early mer-people".

Joan's remarkable collection of photographs show fossils underwater and in the sand, with the unmistakable shape of human skeletons with fish tails.

Now his striking images are on display in Bradford, along with a full-length fossil of what can only be described as a mermaid. It has to be seen to be believed - or does it?

Wander further around the exhibition and you'll come across more images of bizarre creatures - including a monkey with owl-like wings and a flying goat, captured for eternity by taxidermy - as well as photographic evidence of newly-discovered galaxies and a secret order of Scandinavian monks practising miracles.

Welcome to Stranger Than Fiction, the first UK retrospective devoted to Joan's work. Spanning two floors of the National Media Museum, it gathers more than 30 years of work by the internationally renowned photographer, who challenges disciplines such as photography, science and religion. Subversive, deadpan, strange and witty, his works investigate our inclination to believe what we see.

Since the 1980s Joan has been cataloguing his quest to reveal "new and unexplained phenomena", and his hundreds of photographs and documented "evidence" of archaeological findings have been exhibited worldwide. Six of his projects are currently in Bradford following a run at the Science Museum, London.

Joan, 59, says the common reaction to his work is "astonishment". At first glance it looks convincing - alongside photographs of newly-discovered hybrid species are scientific journals, sketches, notes and artefacts from expeditions - but look closer and there are clues suggesting all is not what it seems.

"I use the same technique as a magician - I distract the viewer," says Joan. "My work provides traps, but there are also clues so that when you're looking at something there's a moment when doubt sets in."

Joan has created around 90 works, from sketches to life-size taxidermy. Every aspect of his work is completely constructed, nothing about it is real - not even Joan, who appears in various guises in photographs. Dismissing the notion that the camera never lies, he insists all photographs are constructed to manipulate, whether in a newspaper or a family album. He investigates the truth and reliability of photography, with the overall message that we must question what we see.

"When we see a photograph we tend to believe it is true. It takes more effort to be sceptical," he says. "We accept the authority of a photograph, and a museum setting adds authority. When you see something in a museum you think it must be real."

The mermaid fossil looks real, until you learn it's made from the cast of a human skeleton and dolphin bones. Displayed in a glass case, it is surrounded by photographs of the "discoveries" in what is now a UNESCO World Heritage Site, if you choose to believe the text. The "back story" is that Joan covered the project as a wildlife photographer.

There are photographs of fake fossilised mermaid remains on the walls of a cave. "I left them there, for people to find," says Joan. "I went back recently and they're covered in moss, which makes them look even more authentic."

The exhibition flows in themes, from natural to metaphysical to spiritual. Fauna explores the work of a fictional German professor who spent the early 20th century searching for exceptions to Darwin's theory of evolution. There are black and white photographs of creatures such as a tree-dwelling duck, a rat with a snake for a tail, a flying monkey and a rabbit with huge fangs. Accompanying each image are notes documenting sighting, habits, morphology and date of capture. The winged simian was "found in the Amazon jungle in March, 1944".

An array of items such as binoculars, cameras, handwritten notes, even a photograph of the Professor's children from 1907, are scattered among photographs of weird creatures in flight or scaling trees.

Joan's botanical "findings" are displayed in a National Geographic-style presentation. In the guise of "botanist and physician Carl Linnaeus", he set out to describe new plant species, and the result is a collection of photographs of strange-looking plants - all entirely fabricated from flora and fauna detritus found outside Joan's studio near Barcelona.

Huge sweeping aerial images of mountains and waterfalls turn out to be computer-generated from little models. And in Constellations Joan depicts the night sky and new star systems - except they're nothing more than specks of dust and mosquitos found on his car windscreen.

While some of Joan's photographs are eerie, others are playful. By the time you reach his Miracles photographs you're expected to get the joke. Challenging religion as authority, he presents the premise that he went undercover in a Finnish monastery determined to expose its miracles as a hoax. The 'miracles' he photographed include monks levitating and creating lightning, and an image of Che Guevara in a slice of ham.

Perhaps most remarkable is that Joan is a self-taught photographer. His work has won countless international awards and is in museum collections around the world, from New York's Metropolitan Museum of Art to the Pompidou in Paris. Growing up in Spain under General Franco's regime led Joan to doubt authority and censorship, and his background is in journalism and advertising sparked his quest to challenge how we see the world.

"I got used to lying," he says, a mischievous glint in his eye.

* Joan Fontcuberta: Stranger Than Fiction runs at the National Media Museum until February 5, 2015.