Recessed manhole covers are not just simple "iron lids"; they're more like "Transformers"-flat to the ground when not in use, and quick to open when needed.
Their design incorporates many clever features: rounded edges prevent pedestrians from bumping into them; the interlocking structure between the cover and the manhole ring is precisely calculated to withstand vehicle pressure while preventing rainwater backflow; some models also feature anti-slip textures for a secure grip even in rainy weather.
Even more interestingly, some high-end models have built-in hydraulic buffer devices, opening and closing almost silently, much like a "silent door" from a science fiction movie.
