It took four years to build the museum located on the bank of the Nervion River. Clad in glass, titanium and limestone, the Deconstructivist structure designed by architect Frank O. Gehry appears to defy gravity. It has been hailed as the "greatest building of our time" by one of his colleagues, architect Philip Johnson.

It's hard to believe that such a building could emerge from a draft that looks like mere scribbles. When designing a building, Gehry says that he prepares several sketches in this style — usually spending 15 seconds on each one. He keeps drawing until he feels he's found the right design.

The central atrium, 50 meters high, is flooded with light and offers tons of playful details for the eyes: towers, gallery, corners, ramps, windows and lights. Still, the delightful architecture of the building's interior doesn't steal the show from the art.

Commissioned by the museum and of the permanent exhibition, Richard Serra's sculpture series "A Matter Of Time" is truly monumental. Frank Gehry even had to design a special large hall for the massive pieces made of steel. The "Fish" gallery is 142 meters long and 27 meters wide and comprises different works by Serra, his "Spiral" and "Ellipse" variations, as well as his "Snake" sculpture.

"Maman" is the name of the famous installation by French sculptor Louise Bourgeois on the museum's forecourt. It is made of bronze, almost 10 meters high and weighs over eight tons. The name "Mommy" comes from the fact that the spider carries 26 marble eggs in her sack. "Maman" has many sisters around the world, for example in Tokyo, Saint Petersburg, Ottawa and Seoul.

"Puppy" is the name of the cute, 12-meter-tall flower dog created by the American art superstar Jeff Koons. The artist planted 17,000 marigolds, petunias, begonias and other flowers on the puppy-like statue, and although he is supposed to guard the museum, he is always keen to pose for a photo. In 2015, the museum held a major retrospective of the popular artist.

American artist Mark Rothko was one of the main figures of abstract expressionism. He let the colors on his paintings speak for themselves to achieve a direct communication between the picture and the viewer. The surfaces, shadows, patterns and brush strokes he created were never random. "Untitled" (1952), above, is one of the museum's highlights.

In 2004, the Guggenheim Museum in Bilbao dedicated a large exhibition to pop art pioneer James Rosenquist. The artist, who died in April 2017, is renowned for his huge paintings in which he'd compose seemingly random motifs in oversaturated colors. The artwork in the picture, the "Star Thief," was a loan from the Museum Ludwig in Cologne.

In 2012, the museum featured the largest exhibition ever dedicated to the early work of the Swedish pop artist Claes Oldenburg. His style is characterized by oversized sculptures of everyday objects such as lipsticks, clothespins, cakes, burgers or fries with ketchup made of cheap materials. The show then traveled to Vienna, Cologne and New York.

To celebrate the 20th anniversary of the institution, the Guggenheim Museum was lit up by an impressive 3D light installation that took place over four evenings in October. The performance was created by the British video designers "59 Productions" and took viewers on a journey throughout the history of the museum and art.

The Guggenheim Museum in Bilbao opened 25 years ago. The titanium, sandstone and glass building on the banks of the Nervion River, designed by Canadian architect Frank Gehry, has become a landmark in the Basque capital. It soon became an attraction for more than just architecture- and art-lovers. To date, more than 20 million visitors have brought new wealth to a city formerly in decline.

Abando Indalecio Prieto, Bilbao's main railway station, welcomes travelers with a stunning stained glass mural: 250 square meters (2,700 square feet) of scenes from Basque history. At the left in the picture is the bust of Indalecio Prieto, a prominent politician and socialist who had to flee to exile in Mexico in 1939 during the Spanish Civil War.

Tubular glass tunnels lead to the subway, which connects the city center with its outer districts. The tunnels are called "fosteritos" ("little Fosters"), after Norman Foster, the British architect who designed the metro stations. The metro from San Mames station near the airport takes you into the city. To visit the Guggenheim Museum, it's best to get out at Moyua Station.

A steel sculpture by Basque sculptor Jorge Oteiza has stood on the square in front of the town hall since 2002. Its Spanish name, "Variante Ovoide de la Desocupacion de la Esfera," translates as "Oval-shaped variant of the sphere's idleness." Jorge Oteiza (1908-2003) is considered a pioneer of abstract art in Spain. He was also a philosopher who delved into the Basque soul.

Bilbao, founded in the 13th century, is a commercial hub on the Nervion River, which flows into the Bay of Biscay. It became an industrial city in the 19th century and was long seen as the ugly sister of the seaside resort of Donostia (San Sebastian). But Bilbao's image has been transformed. One more sign of that is the city's tallest building, the Torre Iberdrola (in the center of the photo).

The White Bridge, "Zubizuri" in Basque, was designed by Spanish architect Santiago Calatrava. It was built in 1997 exclusively for pedestrians and is considered by many to be Bilbao's mos-beautiful bridge. Its glass surface had to be covered with a synthetic laminate to keep it from becoming slippery in the rain. In the background stand twin towers designed by Japanese architect Arata Isozaki.

This building by the Spanish architectural firm Coll-Barreu also reflects the city's rapid transformation. The glass facade of the Basque Health Department headquarters, which uses solar energy and absorbs noise, has been a popular photographic subject since being completed in 2004. It is located in the Ensanche area, Bilbao's main shopping district.

Since 2006, the indie, pop and rock festival Bilbao BBK Live has developed into one of the biggest festivals in Europe. This year The Killers, Stromae and the Pet Shop Boys were among the bands that making appearances. Tens of thousands of people gathered to party and dance on Kobetamendi Hill high above the city.

In Basque Country, wearing a beret is a political act to identify with the region. It's said that a citizen from Bilbao wears a hat at home but puts on a Basque beret in Madrid. And, as a little dig against its rival city Donostia (San Sebastian), it's claimed that there, Basques wear berets only at home — and hats in Madrid.

"The Great Week" in Bilbao is devoted to Basque traditions. For nine days, people dance, eat and drink on the streets and squares. Giant puppets, like its official icon Marijaia, make appearances. Next year, the festival begins on August 19 when the traditional rocket, the txupinazo, is set off in front of the Arriaga Theater.

Picos de Europa (literally "peaks of Europe") national park stretches inland from Bilbao. It was founded in 1918 as Spain's first national park. Some 200 peaks higher than 2,000 meters (6,600 feet) offer impressive panoramas for hikers. The famous Way of Saint James, which people from around the world follow to Santiago de Compostela, runs through northern Spain here.

The Atlantic coast about 30 kilometers (18 miles) northeast of Bilbao offers a study in contrasts. There are rocky cliffs and quiet bays; deeply cut fjords and sweeping beaches. A spectacular stone causeway and more than 200 steps lead to the maritime chapel on the islet of San Juan de Gaztelugatxe.

Once a hermitage, the islet is now a pilgrimage site for seafarers and fishermen. Legend has it that John the Baptist left his footprints at the end of the steps. Traditionally, visitors ring the chapel bell three times and make a wish. And as the location of Dragonstone Castle in the seventh season of Game of Thrones, San Juan de Gaztelugatxe is now a must for fans of the series.

From the beaches on the coast, Line 1 on the Bilbao Metro (stations Areeta, Gobela, Bidezabal or Plentzia) takes you back into the city center in about half an hour. After that, it's worth taking a stroll along the riverbank. At dusk, a play of light transforms structures such as the Guggenheim Museum and the White Bridge into spectacular sculptures.