multivariable calculus - Why does $left(int_{-infty}^{infty}e^{-t^2} dt right)^2= int_{-infty}^{infty}int_{-infty}^{infty}e^{-(x^2 + y^2)}dx,dy$?

Why does $$\left(\int_{-\infty}^{\infty}e^{-t^2}dt\right)^2 = \int_{-\infty}^{\infty}\int_{-\infty}^{\infty}e^{-(x^2 + y^2)}dx\,dy ?$$

This came up while studying Fourier analysis. What's the underlying theorem?