In the Caves of St. Pieter, to the south of Maastricht, in the Netherlands. People used to hide in the caves during wartimes, putting burning branches against the walls to heat up the rooms and hallways. The heat from the flames coloured the marl red.
<a href="//commons.wikimedia.org/wiki/User:Anrie" title="User:Anrie">Anrie</a>
CC BY-SA 3.0
Taken by me, <a href="//commons.wikimedia.org/wiki/User:Anrie" title="User:Anrie">Anrie</a>