Bullseye… that they are "immortal" (EA)

I’m still not convinced by that interpretation.

When you see this kind of remark - the first complete series of county maps did not appear until Saxton's immortal project in the 1570's. – it clearly suggests the usage means something like “a piece of research so definitive as designed to last for ages, or to be immortal.”

From the same site, btw, a lovely mention of an old shire name in the UK:

SNOTINGAHAMSCIR, Snotinghamscire, Nottinghamshire: what's in a name? The answer can be a great deal of history. The earliest spelling dates from 1016, but records show that the town after which the shire was named existed as Snotengaham "the village of Snot's people" a century and a half earlier. Exactly who Snot was, we shall alas never know…

I shall struggle to keep a straight face next time I talk to my Nottingham friends

http://www.gwp.enta.net/nottarticle.htm