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The 7 July 2005 London bombings (also called the 7/7 bombings) were a series of coordinated suicide bomb attacks. They were aimed at London's public transport system during the morning rush hour.
At 08:50, three bombs exploded within fifty seconds of each other on three London Underground trains. Then a fourth exploded an hour later at 09:47 on a bus in Tavistock Square. The bombs were packed into rucksacks and set off by the bombers themselves.
A memorial was made in Hyde Park on the 7 July 2009. This had 52 stainless steel pillars, one for each murder victim. They were arranged in four clusters for the four bomb sites. 52 died and over 700 were injured in the attack. Wikipedia