In the summer of 1943, scores in Grimsby and Cleethorpes were killed by the German SD2, or butterfly bomb. Seventy years on, James Rogers sheds some light on one of WWII's lesser-known chapters

It was due to the sheer destructive efficiency of the Luftwaffe air raids on the Grimsby and the Humber region that the government often censored the details of death and injury that occurred there.

Instead, in an attempt to keep morale high – and the enemy unaware of their success – the Home Office ordered that "nothing must leak out". Thus, a mere footnote was often placed in the national papers stating that a 'north east coast town' had been bombed.

It is for this very same reason that to this day few people know of the terror and chaos caused by a new type of bomb that landed in various towns across the country during the second world war.

The first use of these new bombs was on Monday 14 June 1943 when a plague of butterflies landed on Grimsby and Cleethorpes causing terror, chaos and death.

These butterflies were of course the new 'butterfly bombs', or Sprengbombe Dickwandig 2kg (SD2), which were the first ever type of anti-personnel cluster bomb (a bomb specifically intended to kill and maim people, as opposed to buildings or vehicle) to be developed.

Yet it was two key features that made these new cluster bombs so deadly. First, they had rotating wings to slow them and also act as hooks to hang on trees and gutters. Second, they could be armed with one of three fuses: these were the impact fuse, delay fuse and anti-disturbance fuse. When deployed together in a precise and methodical order – such as in Grimsby and Cleethorpes in 1943 – they caused the maximum amount of terror, chaos and death.

The first air raid started at 1.43am. First, high explosive bombs shook the region and incendiaries burned. Vast swaths of fire engulfed the town and many heroic wardens, firefighters and police officers were drawn out into the streets.

It was no coincidence that at this moment the Luftwaffe dropped an estimated 3,000 of the indiscriminate, unpredictable and deadly butterfly bombs that sashayed down silently.

Those armed with an impact fuse made their presence known, exploding as they hit the ground and killed or maimed all in their reach. This caused even more people, such as ambulance crews, military personnel and the men and women of the region to flood the streets and do what they could to assist.

Again not by chance but by intent, the butterflies armed with a delay fuse started to detonate intermittently, causing further death, injury and terror to those who had come to help. Yet this was not the end of the horror.

Unfortunately, more death and destruction was to come that night and over the days, weeks, months and years that followed, as many of the butterflies were armed with an anti-disturbance fuse. This meant they effectively became landmines which could simply lie in wait for their victim.

Very quickly Grimsby became "virtually paralysed" and at "an utter standstill" as the butterfly bomb's symbolic and actual power caused people to live in terror. Many were unable to sleep, instead lying awake wondering when they or their loved ones might fall victim to the butterflies that lay in wait.

Consequently, the pandemonium and terror that these bombs caused led to a public demand for each property in the region to be searched. As a result, a colossal hunt for the butterflies began on 19 June 1943 and consisted of over 10,000 personnel.

However, despite their best efforts, by the time the search was declared complete on 9 July, 114 people has been killed across the region. Staggeringly, this figure equates to over half of all those who lost their lives in the Grimsby area during 37 raids over five years.

• Please note: people have continued to find these bombs over the last 70 years. If an SD2 anti-personnel cluster bomb is found please vacate the area and inform the police. Do not handle this bomb; it is designed to explode with the slightest vibration.

James I Rogers.

James I Rogers is a member of the Centre for Security Studies at the University of Hull. His research focuses on Aerial - Precision & Pariah Weaponry in warfare. For information on the article or if you remember the raid please contact: J.I.Rogers@2012.hull.ac.uk or tweet @JamesIainRogers