It is the largest unbroken volcanic crater in the world. Scientists call this kind of crater a 'caldera'. Imagine a volcano so big that when it ran out of magma underneath, the whole top of the mountain collapsed downwards in one huge piece. What is left is the wall, like the rim of a giant cereal bowl.
When you stand on the rim of Ngorongoro and look down, you can see herds of zebras and wildebeest as tiny dots on the floor far below. There are lions, elephants, hippos in the lake, flamingos turning the water pink, and some of the last wild black rhinos in East Africa - all living together inside the crater.
Because the steep walls are hard to climb, many animals never leave. They are born inside, grow up inside, and live their whole lives in the crater. Scientists call it 'Africa's Garden of Eden'. Whatever season it is outside, there is always water and grass somewhere inside.
Ngorongoro is part of a bigger area called the Ngorongoro Conservation Area, which is a UNESCO World Heritage Site. The Maasai people live and herd cattle on the rim and the surrounding grasslands, just as their families have done for many generations.