It is not uncommon for peat mass and silt marsh to develop subsidence chambers over soil piping and flush site peat bog bursts, as well as drillers, cutters, ditch diggers, pipe layers, animals and general scour erosion from the surface water influx. Generally we found surface waters supplied soil piping on shallow peat Mendip and other sites (MD Stagg DR Weyman LF Curtis MJ Kirkby), but Plynlimon and deep peat hags and slumped pond zones have much better features (MD Newson 1973 1974 1975 IH pipe bursts); there was a small peat soil pipe flushing peat crumbs at artesian state in a marsh in Crafnant ridge North Wales in 1974 (M Spray AD Pinder MD Stagg and a few Landscape students). Usually these features offer in quicksand, clastic fragment seepage (rapid) and slush peat weathering and eroded a great opportunity to fall into a stream as others have noted and be sucked down the pipe if that large, not good for the constitution even if you have climbed the hill at Aberystwyth. Portishead esplanade marsh features have largely washed out now and they were surface backwash formed in fissures, that was my conclusion for Mendip. JAA Jones has better data on deeper pipes and soil sources for water and I suggest this one may be a surface "swallet" style drainage into something below that is substantive in transport capacity. Originally it might have been peat soil drainage subsurface before it opened out. Mike Happy New Year document more if you find them, peat research while political is not very well advanced, in part for safety reasons (JA Taylor UCW 1966 1969) but as you note be careful this is rough territory especially in mist and fog rain.