This was one of the most terrible sieges ever undertaken, for not only were the fortifications enormously strong, but beneath each bastion and outwork, and far extending beyond them, an immense number of galleries had been driven for mines.  At all times soldiers, even the bravest, have found it difficult to withstand the panic brought about by the explosion of mines, and by that underground warfare in which bravery and strength were alike unavailing, and where the bravest as well as the most cowardly were liable at any moment to be blown into the air, or smothered underground.  The dangers of this service, at all times great; were immensely aggravated by the extraordinary pains taken by those who had constructed the fortifications to prepare for subterranean warfare by the construction of galleries.

The miners frequently met underground, breaking into each other’s galleries.  Sometimes the troops, mistaking friend for foe, fought with each other.  Sometimes whole companies entered mines by mistake at the very moment that they were primed for explosion.  They were often drowned, suffocated with smoke, or buried alive.  Sometimes scores were blown into the air.

It was not surprising that even the hearts of the allied troops were appalled at the new and extraordinary dangers which they had to face at the siege of Tournai; and the bravest were indeed exposed to the greatest danger.  The first to mount a breach, to effect a lodgment in an outwork, to enter a newly discovered mine, was sure to perish.  First there was a low rumbling noise, then the earth heaved, and whole companies were scattered with a frightful explosion.

On the 5th of August, a sally made by the besieged was bravely repulsed, and the besiegers, pressing closely upon them, effected a lodgment; but immediately a mine was sprung, and 150 men blown into the air.

On the 20th, the besieged blew down a wall which overhung a sap, and two officers and thirty-four soldiers were killed.

On the 23rd a mine sixty feet long and twenty feet broad was discovered, just as a whole battalion of Hanoverian troops had taken up their place above it.  All were congratulating themselves on the narrow escape, when a mine placed below that they had discovered exploded, burying all in the upper mine in the ruins.

On the 25th, 300 men posted in a large mine which had been discovered, were similarly destroyed by the explosion of another mine below it; and the same night 100 men posted in the ditch were killed by a wall being blown out upon them.

In resisting the attack upon one side of the fortress only, thirty-eight mines were sprung in twenty-six days, almost every one with fatal effect.  It is no detriment to the courage of the troops to say, that they shrank appalled before such sudden and terrible a mode of warfare, and Marlborough and Eugene in person visited the trenches and braved the dangers in order to encourage the men.