http://www.grube-messel.de/de/grube-messel.html (great little animation on this page all German but Chrome/Google will translate)
I shan’t say much and there are no Pictures. Messel Pit is the remains of an extinct volcano. It blew up around 47 million years ago, so that’s 18 million years after the dinosaurs went extinct.
After it blew up, the cone filled in with Ash first then water and finally sediment over several million years (the lake itself took a million years to fill). The entire area was jungle/rainforest. As beasties came alone to drink, or hunt some fell in and the algae that eventually filled the cone became a shale and covered them. Because of the depth of water and the fact that it was still water, the algae broke down in an anaerobic way and formed oil and gas.
For a long time the pit was a mine, then during WW2 it’s production was ramped up using forced labour, then after the war it was closed down because oil and gas could be found easier elsewhere.
This is the insane bit. Even knowing that it contained all this fossil material, it took a lobby of scientists and locals to prevent the place from becoming a landfill in the late sixties.
On reflection the place is now considered so important in terms of the fossil record it contains that it has its own museum, research facility and UNESCO protection. It is eye watering to imagine the time period it covers and the astonishing state of preservation of some of the fossils found. It’s off the beaten track but worth looking at on line and getting behind the science.