Sometimes a fight would take place in a tunnel - 4 feet x 3 feet - and in the dark you didn't know who you were fighting. The only thing was to put your hand over quick to feel if the man had any epaulettes; the Germans used to have epaulettes on the shoulder and we could tell that way. The knuckle knife was very good. One of our officers invented that. It was a specially made knife with a blade about five inches long which was fitted to a brass frame over our hand and strapped to our wrist, so when our fist was closed the knife was at a right angle to our arm. It was silent in action - very handy, especially for raiding galleries.

Lieutenant John Westacott, 2 Canadian Tunnelling Company, quoted in Beneath Flanders Fields by Barton, Doyle & Vandewalle

This quote gives a tiny snapshot of just one of the horrors faced by men of the Tunnelling Companies as they worked beneath the battlefields of WW1, and gives a hint of why they were held in respect by those who knew what their work entailed. I don't know about Churchill or the aristocracy in general, but Field Marshall Haig wrote the following message when the Companies were rapidly disbanded after the Armistice to release desperately-needed skilled labour:

A large number of men are now being withdrawn from the Tunnelling Companies for urgent work at home.

Before they leave the country I wish to convey to the Controllers of Mines and all ranks of the Tunnelling Companies, both Imperial and Overseas, my very keen appreciation of the fine work that has been done by the Tunnelling Companies throughout the last four years.

At their own special work, Mine Warfare, they have demonstrated their complete superiority over the Germans, and whether in the patient defensive mining, the magnificent success at Messines, or in the preparation for the offensive on the Somme, Arras and Ypres, they have shown the highest qualities both as Military Engineers and fighting troops.

Their work in the very dangerous task of removing enemy traps and delay-action charges... has been on a level with their work on the mines.

They have earned the thanks of the whole Army for their contributions to the defeat of the enemy. Their fighting spirit and technical efficiency has enhanced the reputation of the whole Corps of Royal Engineers.

The tunnellers were essentially men working underground, best known for mining operations, but also responsible for building other subterranean works, such as key dugout complexes (some holding up to 600 men) and subways (huge shelters to move and protect large bodies of troops forming up for an attack, sometimes including concealed exits out into no-man's land which would be blown at the key moment to allow troops to pour out close to the enemy line, rather than attacking from their own trenches).

But it was the mining operations that were the best known and most terrible facet of the underground war. Mining (digging under no-man's land towards the enemy trenches) was first used offensively by the Germans on their VII Corps front around Givenchy-Festubert on 20 December 1914, when they detonated 10 'small' (by later standards) explosive charges under the trenches of the Indian Army's Sirhind Brigade. This surprise was followed by an immediate and very successful infantry assault that drove the stunned Indians back 500 yards to their reserve lines.

The British had already been mining defensively to protect against enemy tunnelling, but this attack prompted them to go on the offensive too. The Royal Engineers were already overstretched, so the first efforts involved scouring front-line units for men with mining experience, then looking at home for men such as coal miners, men who had worked on digging the London Underground train system, and sewer workers. In some cases these men had virtually zero military training - in one case a group of Manchester sewer workers went from working in the sewers to tunnelling on the Givenchy front in just five days, forming the nucleus of 170 (Tunnelling) Company, RE, in February 1915. Rapid expansion followed.

Whole volumes have been written on the work of the Tunnelling Companies, but a quick overview of a mining operation is as follows. A deep shaft would first be sunk in the friendly trenches, with a 'shaft house' at the entrance for the air pump etc. Narrow tunnels (known as 'galleries') were then driven underground towards the enemy line. The work was complicated by the fact that it had to be done as silently as possible, as the enemy miners were driving their own galleries in the opposite direction, and each side would be listening out for any sound made whilst working. Once under the enemy position, a mine chamber would be dug out and packed with explosives, which would be fired electrically at a key moment, usually a series of them at the start of a major offensive.

There were obviously huge dangers involved for the men working below ground. These ranged from the natural - such as collapses and carbon monoxide build-up - through to viscous combat with enemy tunnellers. If the other side heard you digging nearby, they might set an explosive charge, called a camouflet, in their own tunnel and set it off to kill or trap you underground. One side might also break into the other's tunnel system and brutal, often hand-to-hand, fighting would ensue, which again would end with charges being set off to wreck the earthworks and kill or trap the enemy miners.

Towards the end of the war, as action became more mobile and less trench-bound in 1918, the Tunnelling Companies moved on to yet more dangerous work above ground, dealing with booby-traps and unexploded shells, including gas. I think, then, it is the sheer amount of danger that they faced that would earn a man involved in tunnelling great respect from those who understood what his work entailed.

Main source is The Western Front Companion by Mark Adkin, which features an excellent section on mining operations.