a calabi-yau manifold is a ricci-flat kahler manifold. it is a longstanding question to understand how the ricci-flat metrics develop singularities when the complex structure degenerates. an especially intriguing phenomenon is that these metrics can collapse to lower dimensions and exhibit very non-algebraic features, and it is challenging to describe the corresponding geometric behavior. in this talk i will discuss an answer to this question for a special class of degenerations. based on joint works with h. hein-j. viaclovsky-r. zhang 2018 and with r. zhang 2019.