  A stellar occultation by Neptune's main satellite, Triton, was observed on 5
October 2017 from Europe, North Africa, and the USA. We derived 90 light curves
from this event, 42 of which yielded a central flash detection.
  We aimed at constraining Triton's atmospheric structure and the seasonal
variations of its atmospheric pressure since the Voyager 2 epoch (1989). We
also derived the shape of the lower atmosphere from central flash analysis. We
used Abel inversions and direct ray-tracing code to provide the density,
pressure, and temperature profiles in the altitude range $\sim$8 km to
$\sim$190 km, corresponding to pressure levels from 9 {\mu}bar down to a few
nanobars.
  Results. (i) A pressure of 1.18$\pm$0.03 {\mu}bar is found at a reference
radius of 1400 km (47 km altitude). (ii) A new analysis of the Voyager 2 radio
science occultation shows that this is consistent with an extrapolation of
pressure down to the surface pressure obtained in 1989. (iii) A survey of
occultations obtained between 1989 and 2017 suggests that an enhancement in
surface pressure as reported during the 1990s might be real, but debatable, due
to very few high S/N light curves and data accessible for reanalysis. The
volatile transport model analysed supports a moderate increase in surface
pressure, with a maximum value around 2005-2015 no higher than 23 {\mu}bar. The
pressures observed in 1995-1997 and 2017 appear mutually inconsistent with the
volatile transport model presented here. (iv) The central flash structure does
not show evidence of an atmospheric distortion. We find an upper limit of
0.0011 for the apparent oblateness of the atmosphere near the 8 km altitude.
