The thermal Hall effect has become a novel probe for extraordinary physics. Recently, a large thermal hall effect was observed in various important insulating materials such as La2CuO4 and SrTiO3. This came as a surprise to the community because in insulators there is no apparent low energy heat carrier except for the phonon, and people conventionally thought the phonon contribution to be small. We investigated the possibility that phonons can produce a large thermal Hall effect through extrinsic mechanisms, and our phonon proposal has been confirmed by subsequent experiments. This talk consists of two parts. In the first part of the talk, we will discuss our phenomenological theory that successfully explained the experimental data of SrTiO3 and predicted more materials that may produce even larger effects. In the second part, beyond phenomenological theory, we will develop a microscopic model for the extrinsic mechanism. We will show how resonance physics may underlie the observed surprisingly large enhancement.