The geological era about which we have the least information is called the Hadean eon.

Here's why:

* Early Earth: The Hadean eon spans from the formation of Earth (about 4.5 billion years ago) to about 4 billion years ago. This is the very early stage of our planet's history.

* No Rock Record: Very few rocks from the Hadean eon survive. The intense heat and volcanic activity of this time would have destroyed most evidence.

* Indirect Evidence: Scientists rely on indirect evidence to understand the Hadean, such as studying meteorites, lunar samples, and models of early planetary formation.

While we have some clues about the Hadean eon, it remains the most mysterious and least understood period in Earth's history.