Where is the hippocampus? See it on a 3D brain you can turn and slice

Where it is

In the medial temporal lobe of each cerebral hemisphere: it is an inward fold of three-layered cortex (archicortex) that bulges into the floor of the inferior horn of the lateral ventricle. [1]
Paired: there is one hippocampus in each hemisphere. Seen whole, the hippocampus and its neighbouring regions form a banana-like curve, and the two sides are joined at their back ends by the commissure of the fornix (the hippocampal commissure). [1]
Deep, not on the outer surface of the brain. Its neighbour along the inner underside of the temporal lobe is the entorhinal cortex, the front part of the parahippocampal gyrus. [2]
Part of the limbic system: with the dentate gyrus and the subiculum it makes up the hippocampal formation. [1]

Consolidates information from short-term memory into long-term memory. [1]
Supports spatial memory, the memory that lets you navigate. [1]
With the entorhinal cortex, it is important for declarative memories (of events and facts), in particular spatial memories, including optimizing them during sleep. [2]
Many neurons in the rat and mouse hippocampus are place cells: each fires in bursts when the animal passes through one particular part of its surroundings. [1]
Long-term potentiation (LTP), the strengthening of connections widely believed to be one of the main ways memories are stored, was first discovered in the hippocampus. [1]
OpenStax groups the hippocampus and amygdala as medial temporal lobe structures that, with the cortex around them, are involved in long-term memory and emotional responses. [3]

Main input: the entorhinal cortex, the main interface between the hippocampus and the neocortex. Its layer II projects mainly to the dentate gyrus and region CA3, its layer III mainly to CA1 and the subiculum. [2]
Main output: the fornix, a C-shaped bundle of nerve fibres that is the major output tract of the hippocampus (it also carries some fibres back in). [4]
Fornix fibres run on through the hypothalamus to the mammillary bodies, which in turn project to the anterior nuclei of the thalamus through the mammillothalamic tract. [4]
It is also connected to areas involved in emotional behaviour: the septal area, the mammillary bodies of the hypothalamus and the anterior nuclear complex of the thalamus. [1]

Its curved band of cortex, the cornu Ammonis, is divided into the subfields CA1 to CA4. [1]

Plural: hippocampi. Also called the hippocampus proper, to set it apart from the wider hippocampal formation. [1]
Cornu Ammonis (“Ammon’s horn”), the old name that survives in the subfield labels CA1–CA4. [1]
The name is Latin for seahorse: the Venetian anatomist Julius Caesar Aranzi (1587) likened the ridge first to a silkworm and then to a seahorse. [1]

In adult humans, the hippocampus on each side has a volume of about 3.5 cm³, compared with 320 to 420 cm³ for the neocortex. [1]
On this model the left hippocampus measures about 43 mm front to back, 22 mm top to bottom and 20 mm side to side (the box it fits in, not its volume); the right one is its mirror image. [5]
Centre of the left hippocampus compared with the centre of the left amygdala: 20 mm further back; 20 mm apart in a straight line. [5]
Centre of the left hippocampus compared with the centre of the left thalamus: 15 mm lower, 15 mm further from the midline; 21 mm apart in a straight line. [5]