While not quite as big as the actual Chunnel (which is over 30 miles long), in butt terms this is sizable and will definitely get you from here to there.
Polished 304 stainless steel tunnel plug. The part that goes in has an inner diameter of 2-5/8" and an outer diameter of 3-1/8" (almost 10" circumference). Tapers to the part that sits out, which has an inner diameter of 2-3/4" and an outer diameter of 3-5/8". Measures 3-7/8" long and weighs over 2-1/2 lbs!