  The class of critical bootstrap percolation models in two dimensions was
recently introduced by Bollob\'as, Smith and Uzzell, and the critical threshold
for percolation was determined up to a constant factor for all such models by
the authors of this paper. Here we develop and refine the techniques introduced
in that paper in order to determine a sharp threshold for the Duarte model.
This resolves a question of Mountford from 1995, and is the first result of its
type for a model with drift.
