6.The new accessibility of land around the periphery of almost every major city sparked an explosion of real estate development and fueled what we now know as urban sprawl．

(宾语从句what we now know as)

现在可以获得这些环绕几乎每个大城市边缘地区的土地，这一可能性激发了一场房地产开发的热潮并造成了我们现在称为城区无计划扩展的现象。

分句1：The new accessibility of land around the periphery of almost every major city sparked an explosion of real estate development and fueled

分句2：what we now know as urban sprawl

分句1嵌套分句2，即分句1是主句，分句2是宾语从句，主句结构经过精简应该是The new accessibility of land sparked an explosion of real estate development and fueled urban sprawl。

本句的理解困难还来自于抽象词fuel，它作为名词的含义广为人知，是能源、燃料，作为动词的时候的意思就可以引申为促使、造成。