From Halopedia, the Halo wiki
The Tanzanian petroglyph is an ancient etching into a mountainside of Tanzania. The etching depicts a specific Forerunner symbol, specifically the base symbol used for Forerunner numbers sometimes referred to as the Chrysopteron. It can be assumed that the etching was created by N'chala after he observed the symbol during the construction of the Portal.
In 2007, a member of the Society of the Ancients learned of the petroglyph while traveling through Tanzania. The member managed to get a picture of the artwork, which can be seen here along with an edited recreation of the symbol. SOTA used the etching as evidence for their hypothesis of ancient alien visitors assisting Humanity.