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2004 was a leap year starting on Thursday of the Gregorian calendar, the 2004th year of the Common Era (CE) and Anno Domini (AD) designations, the 4th year of the 3rd millennium, the 4th year of the 21st century, and the 5th year of the 2000s decade.
2004 was designated as:
- March 2 – A series of bombings occur in Karbala, Iraq, killing over 140 Shia Muslims commemorating the Day of Ashura.
- March 11 – Coordinated bombings at a Cercanías train station in Madrid, Spain, kill at least 192 people.
- March 28 – Hurricane Catarina, the first ever recorded South Atlantic tropical cyclone, makes landfall in Santa Catarina, Brazil.
- March 29 – Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia are admitted to NATO, the largest expansion of the organization.
- August 3 – NASA's MESSENGER spacecraft is launched, with its primary mission being the study of Mercury.
- August 13–29 – The 2004 Summer Olympics are held in Athens, Greece.
- August 22 – Armed robbers steal Edvard Munch's The Scream, Madonna, and other paintings from the Munch Museum in Oslo, Norway.
- August 24 – After departing Domodedovo International Airport in Moscow, Volga-AviaExpress Flight 1303, a Tupolev Tu-134, explodes over Russia's Tula Oblast and crashes, killing all 43 people on board; minutes later, Siberia Airlines Flight 1047, a Tupolev Tu-154 departing the same airport, explodes over Rostov Oblast and crashes, killing all 46 on board. The Government of Russia declares the explosions to have been caused by female Chechen suicide bombers.
- October 8 – Suicide bombers detonate two bombs at the Red Sea resort of Taba, Egypt, killing 34 people and injuring 171, mostly Israeli tourists.
- October 9 – Australian federal election, 2004: John Howard's Liberal/National Coalition Government is re-elected with an increased majority, defeating the Labor Party led by Mark Latham.
- October 19 – A team of explorers reach the bottom of Krubera Cave, the world's deepest cave, with a depth of 2,080 meters (6,824 feet).
- October 29 – European heads of state sign in Rome the Treaty and Final Act, establishing the first European Constitution.
- December 14 – The world's tallest bridge, the Millau Viaduct over the Tarn in the Massif Central mountains, France, is officially opened.
- December 21 – Iraqi insurgents attack a U.S. military base in the city of Mosul, killing 22 people.
- December 26 – The 9.1–9.3 Mw Indian Ocean earthquake shakes northern Sumatra with a maximum Mercalli intensity of IX (Violent). One of the largest observed tsunamis follows, affecting coastal areas of Thailand, India, Sri Lanka, the Maldives, Malaysia, Myanmar, Bangladesh, and Indonesia, killing over 200,000 people.
- December 27 – Astrophysicists from the in Garching near Munich measure the strongest burst from a magnetar. At 21:30:26 UT Earth is hit by a huge wave front of gamma and X-rays. It is the strongest flux of high-energetic gamma radiation measured so far.
- December 30 – A fire in the República Cromañón nightclub in Buenos Aires, Argentina kills 194.
- December 31 – Taipei 101, at the time the tallest skyscraper in the world, standing at a height of 1,670 feet (509 metres), officially opens.
беларуская (тарашкевіца): 2004
srpskohrvatski / српскохрватски: 2004
ئۇيغۇرچە / Uyghurche: 2004