What is Black–Karasinski model?

The Black–Karasinski model (1991) makes the logarithm of the short rate mean-reverting and normal, $d\ln r_t = (\vartheta(t)-\kappa\ln r_t)\,dt+\sigma\,dW_t$: rates stay positive, bond prices have no closed form, and the model is used on trees. It lost ground when rates went negative.