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Minus rail (i.e. most negative the amplifier is capable of).
Say the output is slightly negative or slightly positive. That makes the output of the multiplier slightly positive, but near zero.
An opamp with -1V at it's +input and ~0V at it's -input is driven to the minus rail by it's open-loop gain.
Minus rail multiplied by itself makes positive rail, which makes the difference between -input and +input even bigger, so Minus rail at the opamp output is a stable solution.
Interestingly, for any input above 0V the output will be the square-root of the input. But if the input goes slightly negative, which inverts the feedback polarity, then the output will go to the negative rail.
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