  In the bootstrap percolation model, sites in an $L$ by $L$ square are
initially independently declared active with probability $p$. At each time
step, an inactive site becomes active if at least two of its four neighbours
are active. We study the behaviour as $p \to 0$ and $L \to \infty$
simultaneously of the probability $I(L,p)$ that the entire square is eventually
active. We prove that $I(L,p) \to 1$ if $\liminf p \log L > \lambda$, and
$I(L,p) \to 0$ if $\limsup p \log L < \lambda$, where $\lambda = \pi^2/18$. We
prove the same behaviour, with the same threshold $\lambda$, for the
probability $J(L,p)$ that a site is active by time $L$ in the process on the
infinite lattice. The same results hold for the so-called modified bootstrap
percolation model, but with threshold $\lambda' = \pi^2/6$. The existence of
the thresholds $\lambda,\lambda'$ settles a conjecture of Aizenman and
Lebowitz, while the determination of their values corrects numerical
predictions of Adler, Stauffer and Aharony.
