A plastics infiltration block for infiltration or attenuation purposes is disclosed. The infiltration block has an essentially rectangular parallelepipedal body having pairs of opposed first, second and third faces. At least one inspection channel extends through the infiltration block and inspection channel openings are positioned in opposed third faces of the infiltration block. The area of each inspection channel opening is at least 10% of the size of the area of the third face. The infiltration block can include a single infiltration block element or multiple coupled infiltration block elements. The infiltration block element has an essentially rectangular parallelepipedal body. A grid of duct walls forming substantially parallel ducts extend between each first face and the one or more inspection channels of the infiltration block, and between each second face and the inspection channels of the infiltration block.