In the Circeo National Park, there are some depressions that are periodically flooded due to the accumulation of rainwater or the outcrop of the surface aquifer that form natural pools. This flooded area represents the most evocative environments of this plain forest. Around the pools, or where there is less water, the dominant tree is the oak while within the pools ash trees dominate. It is also the habitat for a great variety of animals that live in a hidden and often unknown microworld: crustaceans (pill-bugs in dry areas, Triops and Artemia in the water), millipedes, annelids and insects.