Intentional Rot

The seriousness of the group was growing with every single step. This wasn't because they were sure that the smell was growing, it was. Sinetad, it was because they could tell they were approaching dangers that they did not yet know. 

Beyond just what they could smell, they could feel the unique darkness and poison energies that came together to make so many different forms of rot. They could tell that the thorny roots or vines, whatever they really were, were feeding on it. That was the strength of whatever controlled this happening. 

The area was literally being reshaped. The violent jungle was no longer a jungle in this area. It was turned in to a rotting swamp for the most part. If Clark was not the one pushing them to explore such things, then they would have stopped already. However, they trusted that he had a good idea of what they might find here. Not to mention that the added challenge will allow them to get stronger.