Close to a British communication trench called Lochnagar Street tunnellers dug a shaft down about 90 feet (27 Metres) deep into the chalk. They then excavated some 300 yards (91 Metres) towards the German lines, placing 60,000 lbs (27 tonnes) of ammonal explosive in two large adjacent underground chambers 60 feet (18 Metres) apart. Two minutes before the attack began, the mine was exploded, leaving the massive crater that we see today.
The reason that it is so large was that the chambers were overcharged. This means that sufficient explosive was used to not just break the surface and form a crater but enough to cause spoil to fall in the surrounding fields and form a lip around the crater. The 15 feet (4.5 Metres) lip created protected the advancing troops from enfilade machine-gun fire from the nearby village of La Boisselle.
For more information on how the crater was formed see the main Military Mining page.