i've found articles to say that the Delaware Aqueduct (right number of letters) is still the longest - 136km - but Wiki says Thirlmere (too many letters)
I actually think that it's neither and it's going to be something else - there's the west-east pipeline in China, - 8074 km, the Enbridge Pipleine 2840............blood vessels (are they 'tunnels') are 1000s of miles if laid out........................