  We study the physics of F-theory compactifications on genus-one fibrations
without section by using an M-theory dual description. The five-dimensional
action obtained by considering M-theory on a Calabi-Yau threefold is compared
with a six-dimensional F-theory effective action reduced on an additional
circle. We propose that the six-dimensional effective action of these setups
admits geometrically massive U(1) vectors with a charged hypermultiplet
spectrum. The absence of a section induces NS-NS and R-R three-form fluxes in
F-theory that are non-trivially supported along the circle and induce a
shift-gauging of certain axions with respect to the Kaluza-Klein vector. In the
five-dimensional effective theory the Kaluza-Klein vector and the massive U(1)s
combine into a linear combination that is massless. This U(1) is identified
with the massless U(1) corresponding to the multi-section of the Calabi-Yau
threefold in M-theory. We confirm this interpretation by computing the one-loop
Chern-Simons terms for the massless vectors of the five-dimensional setup by
integrating out all massive states. A closed formula is found that accounts for
the hypermultiplets charged under the massive U(1)s.
