  The 3D fine structure of the solar atmosphere is still not fully understood
as most of the available observations are taken from a single vantage point.
The goal of the paper is to study the 3D distribution of small-scale
brightening events ("campfires") discovered in the EUV quiet Sun by the Extreme
Ultraviolet Imager (EUI) aboard Solar Orbiter. We used a first commissioning
data set acquired by the EUI's High Resolution EUV telescope on 30 May 2020 in
the 174 {\AA} passband and we combined it with simultaneous data taken by the
Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory in a
similar 171 {\AA} passband. The two-pixel spatial resolution of the two
telescopes is 400 km and 880 km, respectively, which is sufficient to identify
the campfires in both data sets. The two spacecraft had an angular separation
of around 31.5 degrees (essentially in heliographic longitude), which allowed
for the 3D reconstruction of the campfire position. These observations
represent the first time that stereoscopy was achieved for brightenings at such
a small scale. Manual and automatic triangulation methods were used to
characterize the campfire data. The height of the campfires is located between
1000 km and 5000 km above the photosphere and we find a good agreement between
the manual and automatic methods. The internal structure of campfires is mostly
unresolved by AIA; however, for a particularly large campfire, we were able to
triangulate a few pixels, which are all in a narrow range between 2500 and 4500
km. The low height of EUI campfires suggests that they belong to the previously
unresolved fine structure of the transition region and low corona of the quiet
Sun. They are probably apexes of small-scale dynamic loops heated internally to
coronal temperatures. This work demonstrates that high-resolution stereoscopy
of structures in the solar atmosphere has become feasible.
