  Tracking nanoparticle movement is highly desirable in many scientific areas,
and various imaging methods have been employed to achieve this goal.
Interferometric scattering (iSCAT) microscopy has been particularly successful
in combining very high spatial and temporal resolution for tracking small
nanoparticles in all three dimensions. However, previous works have been
limited to an axial range of only a few hundred nanometers. Here, we present a
robust and efficient strategy for localizing nanoparticles recorded in
high-speed iSCAT videos in three dimensions over tens of micrometers. We
showcase the performance of our algorithm by tracking gold nanoparticles as
small as 10 nm diffusing in water while maintaining 5 {\mu}s temporal
resolution and nanometer axial localization precision. Our results hold promise
for applications in cell biology and material science, where the
three-dimensional motion of nanoparticles in complex media is of interest.
