This little culvert, found near Droylsden, is around 350m long and built of stone and brick. I went in from the outfall, after a stoopy 4f beginning it opened up slightly and allowed a more comfortable walk. The shape changed from circular to pill to arch, the height varied from 4-5ft. The central section shrank to 3-4ft egg for a thankfully short distance.
After was more arch which eventually turned to box shape at a small waterslide, probably the more interesting part of the entire drain.
I was close to turning back after this point as the stooping was doing me in, but a few more minutes gave a glimmer of light in the distance. The infall eventually greeted me where I sat and chilled for a bit before going back in for the return trip.
Not the most interesting or comfortable to explore of drains, but full of shape changes and stone is always a bonus, especially when compared to modern RCP.