Serta's Serene Sky 12"" Firm Mattress offers Serta's renowned comfort at an unbeatable value. Cool Twist Gel Foam works to create a cool sleep surface which is essential for a deeper sleep while the 805 Individually Wrapped Coil System provides contouring cradling support. Finally, BestEdge Foam Encasement ensures that you will not have a sinking feeling at the edge of the mattress.