The Lyman- \alpha ( hereafter Ly \alpha ) forest is a probe of large-scale matter density fluctuations at high redshift , z > 2.1 .
It consists of H i absorption spectra along individual lines-of-sight .
If the line-of-sight density is large enough , 3D maps of H i absorption can be inferred by tomographic reconstruction .
In this article , we investigate the Ly \alpha forest available in the Stripe 82 field ( 220 \mathrm { deg ^ { 2 } } ) , based on the quasar spectra from SDSS Data Release DR16 .
The density of observed quasar spectra is 37 \mathrm { deg ^ { -2 } } with a mean pixel signal-to-noise ratio of two per angstrom .
This study provides an intermediate case between the average SDSS density and that of the much denser but smaller CLAMATO survey .
We derive a 3D map of large-scale matter fluctuations from these data , using a Wiener filter technique .
The total volume of the map is 0.94 \mathrm { h ^ { -3 } Gpc ^ { 3 } } .
Its resolution is 13 \mathrm { h ^ { -1 } Mpc } , which is related to the mean transverse distance between nearest lines-of-sight .
From this map , we provide a catalog of voids and protocluster candidates in the cosmic web .
The map-making and void catalog are compared to simulated eBOSS Stripe 82 observations .
A stack over quasar positions provides a visualization of the Ly \alpha -quasar cross-correlation .
This tomographic reconstruction constitutes the largest-volume high-redshift 3D map of matter fluctuations .