Three-loophole, frontal-firing heavy machine gun pillbox. What you see on the photo is an exposed back of the structure. The cracks in the massive concrete slab are a result of detonation of explosive material which had been used to blow off the armored housings of Maxim machine guns (they are on the opposite side).
This two-storey bunker would normally sit deeper – the ground level would obviously be where the entrance is. The slit, or opening, to the left of the photo served an important purpose: it allowed the blast of an explosive charge, or grenades, thrown into the entrance, to dissipate. A small loophole further to the left housed an extra light machine gun to protect the entrance. Just below there's a metal pipe sticking out – it was an air intake. All of the pillboxes required a constant supply of fresh air – and lots of it – once the guns started to sing their tune the amount of smoke inside the bunker would make the life of the crew unbearable so proper ventilation systems had to take care of efficient air exchange.
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