Below it, denser salt water intrudes from the ocean.

At the boundary itself, bacteria that live on the boundary between different chemical environments produce hydrogen sulfide and other compounds as byproducts of their metabolism.

Hoyo Negro is deep enough that it sits well below the halocline in the saltwater zone.

The water chemistry of the deep salt water layer in the cave system is different from the fresh water above — more stable, less variable, less affected by the surface processes that introduce organic acids and other compounds that attack biological material.

The consistent temperature of the deep cave water — around twenty-five degrees Celsius, with very little variation — also limits the bacterial activity that is the primary driver of DNA degradation.