In reply to UKC News:
I see; answering myself, the route is presumably named after a 'sea' on the Moon, which someone - presumably someone American - called Sea of Tranquility despite the fact it was apparently named as Mare Tranquillitas, thus indicating both the correct spelling and the reason why all in one.
Ah well. You can't expect any better from Americans, I suppose, but I do wonder why they got to misspell it, given that (I'm guessing from the Latin names, anyway) it had been named before America even existed.
jcm