Type 583a / M 178. Fire control post for medium and heavy batteries. Heerenduin, Ijmuiden, The Netherlands. 600 gun site for 5cm tank gun. In the background there is a 630 Embrasured MG emplacement with armor plating. Air raid shelter. Brasschaat Airfield, Belgium. Type L483 central radio transmitter bunker. Spaandam, The Netherlands. Cramond Island World War II submarine defense boom. Dutch double pyramide bunker. Military casemate type 623. West of Koudekerke, The Netherlands. SK observation tower. Fliegerhorst, Hemiksem, Belgium. R636 fire control post. Leffrinckoucke/Zuydcoote, Plage L'Ouest de Zuydcoote, France. VF observation bunker, Raversijde, Oostende, Belgium. 134 ammunition bunker. Patentestraat, Lissewege, West Vlaanderen, Belgium.

Hitler knew the Allied invasion on the Atlantic coast would come eventually, so all along the western edge of Europe — from Spain to Scandanavia — he built a series of fortifications called the Atlantic Wall. In addition to minefields, workers were ordered to build a series of massive concrete bunkers designed to house troops and guns. Decades later, a group of those structures still exist.

Photographer Jonathan Andrew came across these relics while driving around the Netherlands on assignment. With walls up to 9 feet thick, some have withstood the test of time only to be covered in graffiti or converted into livestock barns by local farmers. Andrew was drawn to the bunkers as photo subjects by their strange architectural design.

"It was a visual thing first, but I was also fascinated with their history," says Andrew, a commercial photographer who was born in England but now lives in Amsterdam.

He first set out to document the bunkers in the winter of 2008-2009. His commercial assignments had slowed along with the economy and he had a chunk of time on his hands. Armed with an old 4x5 camera that he outfitted with a Phase One digital back, he started picking off bunkers in Holland, France and Belgium.

Instead of shooting the structures during the day — which can often lead to flat photos — he decided the buildings looked better just as the last rays of sun disappeared at dusk. He liked the evening sky, but also liked that he could bring his own artificial lights and control what architectural features were highlighted.

To make the pictures, Andrew set up his camera on a tripod, opened the shutter and walked around the building, popping off a flash dozens of time like a light-painter. To keep from being seen in the picture, he would wear black clothing from head to toe. Black shoes, black gloves, a black balaclava and a black backpack to hold the battery he needed to fire off his 1200 watt flash.

"I look like a ninja assassin," he says.

He prefers the winter for these shots – not only because he has more free time, but also because the sun goes down earlier and the weeds that sometimes cover the buildings recede. The problem with the winter is that it often rains or snows, both of which will ruin the shot because they catch the light of the flash.

In addition to France, Belgium and the Netherlands he's also shot in Scotland, and this coming week he's off to shoot the bunkers in the Channel Islands between England and France. He's shot one Dutch-built bunker and the Cramond Island submarine defense boom, but the majority of the structures are German.

The challenge these days is to find bunkers that are unique. He says he's come across a lot of similar structures and wants to make sure that whatever he shoots adds visual variety. After becoming a viral hit in 2011, the project got mostly shelved because Andrew was busy with paying assignments (the bunkers work is purely personal). But this year he's been hitting the road more frequently, fueled by a renewed interest in the blogosphere.

"It's been great to have a resurgence and it's motivated me to really take the project seriously again," he says.