After the German attack at the Bluff, the V Corps commander, Lieutenant-General Herbert Plumer, decided that it would be more advantageous to make a retaliatory attack at St Eloi, about 1 mi (1.6 km) to the west.
Since an attack on 14 March 1915, the Germans had held a salient 600 yd (550 m) wide and 100 yd (91 m) deep around the Mound, against which an attack had been planned in November 1915.
The salient was on a slight spur that ran down from the higher ground around Ypres, which gave a commanding view over the British lines.
An attack on the Mound required mines to be sunk much deeper and work by the 172nd Tunnelling Company had begun in August 1915.
Three shafts were dug 50–60 ft (15–18 m) deep and by  November, when the Germans blew a mine at the Bluff, a line of shallow galleries had been dug.
Work on the deep mines continued for a possible operation in February 1916; eventually it was decided to dig six galleries from the deep shafts.
After another German mine explosion at the Bluff in January, work on the shallow mines was stopped and all efforts were made to finish the deep galleries at St Eloi.

German miners could be heard above the deep galleries, which showed that the galleries had been advanced under the German lines but the British deep mines, D1, D2, H1, H4 and F appeared to be safe from discovery.
Work was stopped on mine I, the furthest west, which was thought to be most vulnerable to exposure and on 10 March 1916, the Germans blew a camouflet which collapsed 20 ft (6.1 m) of the gallery.
The British blew their own camouflet on 24 March, which collapsed more of the gallery and a charge was placed 240 ft (73 m) along, even though the German trenches were 420 ft (130 m) away.
F gallery was dug at 38 ft (12 m) and then stopped, when it ran into German defensive mines about 100 ft (30 m) from the German lines.
Work still went on in the higher galleries and the British tunnellers entered two German galleries and demolished them.
There were four central galleries, two laid from shaft D and two from shaft H. Mines 1–6 contained 1,800 lb (820 kg), 31,000 lb (14,000 kg), 15,000 lb (6,800 kg), 13,500,000 lb (6,100,000 kg) 12,000 lb (5,400 kg) and 600 lb (270 kg) of ammonal. (
For most of the time, the British preparations were obstructed by an efficient German counter-mining effort but the British carried away excavated earth in sacks by hand, dumped it in dips unseen and then camouflaged the dumps.)
From Kemmel Hill, a hollow south of St Eloi could be observed and was thought to be a German assembly area.
No artillery-fire was directed on the area but a code word was arranged at which the guns were to open rapid fire.
After the German attack at the Bluff, the V Corps commander, Lieutenant-General Herbert Plumer, decided that it would be more advantageous to make a retaliatory attack at St Eloi, about 1 mi (1.6 km) to the west. Since an attack on 14 March 1915, the Germans had held a salient 600 yd (550 m) wide and 100 yd (91 m) deep around the Mound, against which an attack had been planned in November 1915. The salient was on a slight spur that ran down from the higher ground around Ypres, which gave a commanding view over the British lines. An attack on the Mound required mines to be sunk much deeper and work by the 172nd Tunnelling Company had begun in August 1915. Three shafts were dug 50–60 ft (15–18 m) deep and by  November, when the Germans blew a mine at the Bluff, a line of shallow galleries had been dug. Work on the deep mines continued for a possible operation in February 1916; eventually it was decided to dig six galleries from the deep shafts. After another German mine explosion at the Bluff in January, work on the shallow mines was stopped and all efforts were made to finish the deep galleries at St Eloi.
German miners could be heard above the deep galleries, which showed that the galleries had been advanced under the German lines but the British deep mines, D1, D2, H1, H4 and F appeared to be safe from discovery. Work was stopped on mine I, the furthest west, which was thought to be most vulnerable to exposure and on 10 March 1916, the Germans blew a camouflet which collapsed 20 ft (6.1 m) of the gallery. The British blew their own camouflet on 24 March, which collapsed more of the gallery and a charge was placed 240 ft (73 m) along, even though the German trenches were 420 ft (130 m) away. F gallery was dug at 38 ft (12 m) and then stopped, when it ran into German defensive mines about 100 ft (30 m) from the German lines. Work still went on in the higher galleries and the British tunnellers entered two German galleries and demolished them. There were four central galleries, two laid from shaft D and two from shaft H. Mines 1–6 contained 1,800 lb (820 kg), 31,000 lb (14,000 kg), 15,000 lb (6,800 kg), 13,500,000 lb (6,100,000 kg) 12,000 lb (5,400 kg) and 600 lb (270 kg) of ammonal. (For most of the time, the British preparations were obstructed by an efficient German counter-mining effort but the British carried away excavated earth in sacks by hand, dumped it in dips unseen and then camouflaged the dumps.) From Kemmel Hill, a hollow south of St Eloi could be observed and was thought to be a German assembly area. No artillery-fire was directed on the area but a code word was arranged at which the guns were to open rapid fire.
Actions of St Eloi Craters