  We report on the evolution of the X-ray emission of the accreting neutron
star (NS) low mass X-ray binary (LMXB), MXB 1659-298, during its most recent
outburst in 2015-2017. We detected 60 absorption lines during the soft state
(of which 21 at more than 3 $\sigma$), that disappeared in the hard state
(e.g., the Fe xxv and Fe xxvi lines). The absorbing plasma is at rest, likely
part of the accretion disc atmosphere. The bulk of the absorption features can
be reproduced by a high column density ($log(N_H/cm^{-2})\sim23.5$) of highly
ionised ($log(\xi/erg~cm~s^{-1})\sim3.8$) plasma. Its disappearance during the
hard state is likely the consequence of a thermal photo-ionisation instability.
MXB 1659-298's continuum emission can be described by the sum of an absorbed
disk black body and its Comptonised emission, plus a black body component. The
observed spectral evolution with state is in line with that typically observed
in atoll and stellar mass black hole LMXB. The presence of a relativistic Fe
K$\alpha$ disk-line is required during the soft state. We also tentatively
detect the Fe xxii doublet, whose ratio suggests an electron density of the
absorber of $n_e>10^{13} cm^{-3}$, hence, the absorber is likely located at
$<7\times10^4 r_g$ from the illuminating source, well inside the Compton and
outer disc radii. MXB 1659-298 is the third well monitored atoll LMXB
showcasing intense Fe xxv and Fe xxvi absorption during the soft state that
disappears during the hard state.
