the sahara desert is __the sea of death

what we call
前面是the sahara desert is ，后面应该加宾语从句
而that作一般陈述句的引导词，这里应该用what，因为
后面引导的是一个名称，what对应先行词the sahara desert

选 what we call
名词性从句 what 引导 what we call the sea of death的名词性从句做表语

the Sahara desert is what we call the sea of death

what we call
这里what是指(用作关系代词)所...的事物(或人)
例句：I don't care what she thinks.
我不管她怎么想。

That what is called