This picture shows a group of Canadian soldiers cleaning their giant BL-15 Howitzer. This artillery piece was designed and built in Britain and was tasked with smashing deep German underground fortifications by dropping gigantic shells on the enemy. It had a range of about 6 miles and was used exclusively on the western front.
The shells were longer than a normal sized man. Below is a picture of a German soldier posing next to an unexploded shell that had landed near his trenches.