Alkai oak, which grows near Paalkiai mound, is the thickest oak in the regional park. The circumference of the oak trunk reaches as much as 5.12 m, height—18 m, and it is 300 years old. This century-old oak in 2009 was shown by S. Kanevičius, a resident of Kazokiškės. The place where this protected oak grows is special. It is a chain of natural hills called the Alkai Mountains (the old shrines were called alkai). The holy Alkai forest which is mentioned in the 16th C. sources once grew here. Therefore, the discovered oak was named Alkų oak