Abstract: Multilayer polyethylene (PE) geomembrane liners for unconventional thermal conditions and methods of making them are disclosed. The liners have N co-extruded stacked layers, each layer being made from a PE masterbatch composition comprising a given amount of PE resin and additives, and of a PE metallocene based resin free of acid neutralizer compounds. Also disclosed are customized multilayer PE geomembrane liners of N co-extruded stacked layers, each layer being made from a PE masterbatch comprising a specific additive providing specific behaviour property to the layer; wherein at least two of the N layers comprise a different specific additive in order to provide a geomembrane liner having at least two layers with different behaviour property. The liners have a water-bath aging performance for about 6 months immersion at about 80° C. per ASTM D5322—modified for water, such that about 80% of high pressure OIT value retained per ASTM D5885.