A Grand Slam being released by 617 Squadron’s Flg Off Phil Martin on 19 March 1945, in the successful attack on the Arnsberg Viaduct. [IWM CH 15375/Wikipedia] 

617 Squadron’s reputation as the RAF’s crack bomber squadron was cemented towards the end of the war when it was chosen to drop the first ‘Grand Slam’ bomb. This was the biggest conventional bomb used during the war, and weighed in at 22,000 lbs. It had been designed by Barnes Wallis to create an ‘earthquake’ effect which would destroy buildings or structures in the area without necessarily hitting them directly.
On the morning of 13 March 1945 the Grand Slam was tested at the same New Forest bombing range which had been used in the summer of 1943 to see whether the ‘bouncing bomb’ used on the Dams Raid would be effective when dropped on land. (In fact, it caused a trail of debris to be thrown up which would damage low-flying aircraft, so it was never used.) The Grand Slam test drop, on the other hand, was very successful, leaving a crater 70 feet deep and 130 feet in diameter.
That afternoon at Woodhall Spa, two specially adapted 617 Squadron Lancasters were loaded with Grand Slams and a further 18 with the 10,000 lb Tallboys, and set off for the Bielefeld Viaduct, an important railway target which had withstood several earlier attempts to destroy it. The two Grand Slams were in aircraft piloted by 617 Sqn CO Group Capt Johnny Fauquier and flight commander Sqn Ldr Charles (‘Jock’) Calder. However, when they reached the target it was completely enveloped in cloud, so bombing was impossible. The squadron returned to base, although as a precaution, given their huge load, Fauquier and Calder landed at the emergency landing strip at Carnaby with its much longer runway.
The next day, Fauquier and Calder’s aircraft were reloaded with Grand Slams, ready for another operation against the same target. Fourteen Lancasters were loaded with Tallboys. When Fauquier’s aircraft developed a mechanical fault before take off he jumped out and ran towards Calder. Calder guessed correctly that his aircraft might be commandeered by his superior officer and ignored the gesticulations from the runway.
Calder dropped his Grand Slam at the viaduct at 1628. Witnesses described it as looking something like a telegraph pole as it fell, and that it was considerably bigger than anything that had been seen before. The airborne shockwave was felt 3km away. When combined with the effect of the Tallboys dropped around the same time, the effect was as awesome as Wallis had predicted and five arches on the viaduct were destroyed.
The damage done in the test drop in the New Forest is now being investigated by archaeologists for the first time since the war, according to a recent report in The Independent:

The New Forest National Park Authority’s current geophysical survey and historical investigation into Grand Slam is part of a wider project researching and surveying the park’s often unappreciated wartime role. Quite apart from Grand Slam, the New Forest was used as a test site for the first Barnes Wallis bouncing bombs, the development of the ‘Tallboy’ predecessor of Grand Slam, as well as early demonstrations of the Churchill tank. The forest was also home to nine wartime airfields, many of which played a key role in D-Day.

41 more Grand Slams were dropped between 13 March 1945 and the end of the war. If they had been available earlier in the war, by how much would it have been foreshortened?
Jock Calder was one of 617 Squadron’s most distinguished pilots in the last few months of the war. He died in 1997.

[Hat tip: Graeme Stevenson]