The Somme is remembered for slaughter with men machine-gunned in their tens of thousands. But it also featured a bold attempt at breaking the deadlock - a form of warfare revolutionised by an unlikely group of British sewer workers.
The giant Lochnagar Crater, south of the French village of La Boisselle, has never been filled.
The chalky soil of the 300ft diameter depression is now covered in grass and is overlooked by a wooden cross which stands as a memorial to all the dead in the bloodbath that was the Somme.
But this hole is also testament to an attempt to master the art of mining and reinvent it for the age of the machine gun. A 1,000ft tunnel was filled with 27 tonnes of ammonal explosive and the resulting explosion threw debris 4,000ft into the air.
It was the largest of the series of mines [the total is variously recorded as either 17 or 19] laid across the line of the Allied attack that day. The series of blasts was at that point the largest manmade sound ever made.
The revolution in using tunnelling for war had started in February 1915 with the formation of an extraordinary underground unit.
The first 18 men were not typical Army recruits.
Many were over 40. A few were white-haired and toothless. Most were small - less than 5ft 4in tall, at a time when the average height was 5ft 8in. They had strong backs and calloused hands....Cont