British Combat Engineers/Sappers conduct the deadliest op of WW1.

via Today in WW1 Tumblr Page.

June 7 1917, Messines–British sappers had been digging under the German lines in the Flanders clay since 1915, and had been specifically preparing for an attack on Messines since early 1916.  The ridge at Messines (although really more the crest of a gentle slope) dominated the southern flank of the Ypres salient.  By the end of May 1917, the sappers’ work was more than ready, nineteen mines had been placed under the German lines, and the British Second Army was ready to take advantage of the explosion that would result.

At 3:10 AM on June 7, the nineteen mines were detonated within 20 seconds of each other.  Taken together, it was the largest deliberate explosion until Hiroshima, and is the most deadly non-nuclear explosion in history, killing over 10,000 Germans in the front lines.  A German observer recalled that nineteen mushroom clouds rose up slowly and majestically out of the ground and then split into pieces with a mighty roar, sending up multicolored columns of flame mixed with a mass of earth and splinters into the sky.

 The explosion destroyed many of the long-standing landmarks on the battlefield, including Hill 60, site of the first major sapping operation.  The explosion was reportedly heard as far away as Dublin; in Lille, it was mistaken for an earthquake.

A creeping barrage followed to pound whoever survived the explosion into submission.  The British infantry and tanks that followed afterwards were able to seize their objectives of 1-2 miles distant within hours, their largest obstacle often the large craters made by the initial explosion.

The operation was mainly a tactical one; there were no real plans to attempt a major breakthrough. Although Haig was eager to push the attack beyond the initial objectives, commanders on the ground were less sanguine as to their chances, and any opportunity soon slipped by.  The next week saw the usual deadly round of counterattacks, but the British held their ground.

From Haig’s perspective, the victory at Messines was highly important for two reasons.  First, it distracted the Germans from the French mutinies, as Pétain requested of him (though there is no indication the Germans were aware of them).  Secondly, it secured the British right flank at Ypres, where Haig was eager to launch another offensive later in the summer.

Two of the mines laid were never detonated. One was set off by lightning in 1955; thankfully the only casualty was a single cow.  The other still remains unexploded under the battlefield.

Note, Wiki page is here and they're saying that the largest bomb was just under 100,000 pounds.