what is at the bottom of sparta's social group?
http://www.pbs.org/empires/thegreeks/educational/lesson1.html
Scroll down to Social Structure and then look in the second column (Social Structure of Sparta). You'll find it.
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At the bottom of Sparta's social group was the group of people known as the helots.
To find this information, you can visit the provided website link and scroll down to the section titled "Social Structure" in the second column. There, you will find a detailed explanation of the social structure of Sparta, which includes the mention of the helots as the lower class.