Venient annis saecula seris,
quibus Oceanus vincula rerum
laxet, et ingens pateat tellus,
Tethysque novos detegat orbes
nec sit terris ultima Thule.
I have my doubts with respect to the grammatical function of terris (it results in slightly different meaning; in some of those, for example, Thule is not considered to be part of the terrae):
- ablative of place ("farthest in lands") - but without preposition it makes me wonder.
- ablative of separation ("farthest from lands". i.e. "from/of all the lands, Thule isn't the farthest") - I would usually, maybe wrongly, expect the genitive in those cases.
- ablative of separation ("farthest from lands". i.e. a very remote place distance-wise from the "lands")
- dative ("farthest for/to the lands")