Algae coating the damp cracks in the wall. There also looks to be a cover next to the hole that potentially slides across to seal it - I would assume this was for ventilation purposes.

Green Stains

Algae coating the damp cracks in the wall. There also looks to be a cover next to the hole that potentially slides across to seal it - I would assume this was for ventilation purposes.
Credit
Toby J
Taken
03/09/2017
Camera
Nikon D7100
ISO
160
Shutter
130 second
Aperture
ƒ/6.3
Focal Length
20mm
Lens
Nikon AF-S DX NIKKOR 16-85mm f/3.5-5.6G ED VR
License
Copyrighted
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Jinshan Ironworks 台灣煉鐵金山廠