The present invention concerns a plastics infiltration block adapted for combining multiple infiltration blocks into an underground construction for infiltration or attenuation purposes. The infiltration block has an essentially rectangular parallelepipedal body having pairs of opposed first, second and third faces. Through the infiltration block at least one inspection channel extends, with inspection channel openings 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 in which it is positioned. The infiltration block comprises a single infiltration block element or multiple couple infiltration block elements, wherein one or more inspection channel sections of the one or more inspection channels are delimited by a single or multiple coupled infiltration block elements. At least one infiltration block element is of a one-part plastic construction having an essentially rectangular parallelepipedal body with side body parts essentially on opposite sides of the one or more inspection channel sections, and with at least one intermediate body part situated between the side body parts. Side body parts form the first faces of the infiltration block and intermediate body parts form second faces of the infiltration block. The side body parts and one or more intermediate body parts of said one-part plastic construction infiltration block element are essentially formed by a grid of duct walls, which duct walls form essentially parallel ducts extending between each first face and the one or more inspection channels of the infiltration block element, and between each second face and the one or more inspection channels of the infiltration block element. The total area covered by the one or more inspection channel openings is at least half of the size of the area of the third face of the infiltration block in which this one or more inspection channel openings are positioned.