Complex sentence: The purpose of a fogou is now unknown and there is little evidence to suggest what it might have been although their central location in settlements and the work which must evidently have gone into constructing them is indicative of their importance which may have been religious.
Split into simpler sentences: The purpose of a fogou is now unknown, and there is little evidence to suggest what it might have been. Their central location in settlements and the work which evidently went into constructing them is indicative of their importance, which may have been religious.