Geological fieldworks, underground in the Estonian oilshale mine to study the variable mineralogical and chemical composition and microstructure in the different layers of the oil shale profile
<a href="//commons.wikimedia.org/wiki/User:Peeterpaaver" title="User:Peeterpaaver">Peeter Paaver</a>
CC BY 4.0
<span class="int-own-work" lang="en">Own work</span>