[Tuor of Gondolin]

Middle-earth mysteries:

What was the cause of the Enchanted River in Mirkwood
being "enchanted."?

About how strong in numbers and where were the Rangers of the
Third Age situated (I believe Michael Martinez postulates
they're being based in the angle of the Mitheithel and
Bruinen). And did they mostly emigrate to west Arnor or
Gondor in the Fourth Age, and what problems might their
interactions with Gondorites cause?

What exactly was Radagast doing for hundreds of years, and
where was he based after he abandoned Rhosgobel?

As for the entwives, in Letters Tolkien tentatively suggests
they perished when their lands were overrun (I don't have the
book with me. I'll try to get the information later).