From this videoconference talk from 1 March 2019, Ajahn Anan discusses the four imponderables that the Buddha taught.

There are four things that the Buddha taught should not be thought about, because they can drive one crazy:

1) The knowledge and power of a Buddha.

2) The abilities of those with deep concentration (jhana).

3) The details of how karma works.

4) Speculation about the universe (origin, etc.).

An excerpt: "It's beyond the ability of humans to be able to think about and comprehend... And to think too much about it, one’s views may become distorted."

Note: One can read Dhamma talk transcripts at the Wat Marp Jan 'Reflections' page. These include questions and answers with Ajahn Anan, which are not in the audio files. One can read this talk's transcript here: https://watmarpjan.org/en/the-four-imponderables/