At issue is the Thirty Meter Telescope’s planned location on Mauna Kea, a dormant volcano worshiped by some Hawaiians as piko, that connects the Hawaiian Islands to the heavens. 

请教各位大神， 

这里， that 是在修饰piko么？  是个定语从句还是个插入语呢？ 

为什么前边 有个逗号呢，？   that可以引导非限定定语从句么？