Peat substrate which, thanks to its low pH (5-6), considerably reduces bacterial growth, even with 90% humidity. It is therefore ideal for reproducing a natural environment with a high level of humidity.
It can also be placed on top of a draining substrate such as Hydroclay or clay balls. Remember to use a layer separator like Repti Mesh between the two.
If you wish to place real plants in your terrarium, also remember to add a nutrient soil at their base.