Are there any descriptions of the deathless/unborn in any tradition of Buddhism?

Most things that we read about in Buddhism describe aspects of the conditioned world: Three Marks of Existence, The Skandhas, The Hindrances, Defilements, and the practice to overcome these and gain insight into the true nature of reality and enter the unborn to gain final freedom from the cycle of birth and death. I would like to read about the unborn: what it is, how/why conditioned existence comes into being (perhaps from the unborn?), etc. I think this is one of the things which the Buddha left unanswered, but did he describe it indirectly perhaps? Are there descriptions in the sutras (from any school of Buddhism) about the unborn/deathless state?