Short answer
The hippocampus is the seahorse cut into the wave-edged medallion on the back of the Omega Seamaster Diver 300M and its 2022 variants. It is the single detail that separates a top replica from a cheap one, because the seahorse is drawn in raised relief with fine fins, a coiled tail and a shadow line that only a properly cut die can hold. If the seahorse looks flat, gray and shadowless when you tilt the case against a lamp, the engraving was stamped from a worn tool or printed as a decal. If the fins, tail coils and wave border stay crisp under raking light, the build uses a fresh die or a laser pass.
Why 2022 models get judged on this detail
Seamaster Diver 300M casebacks from this period keep the same layout: a circular medallion with the hippocampus in the center, a wave border running around it, and the model text ringed outside. Because the layout is fixed and widely photographed, replica makers copy the shape but rarely the depth. Depth is where the tell sits. Genuine medallions are struck so the seahorse stands proud of the field with a consistent wall thickness along every edge. Most replicas cut a shallow relief that reads as a silhouette at arm's length and as a smear under magnification.
What you need
- A 10x jeweler loupe or a phone macro lens
- A single desk lamp you can angle low
- A microfiber cloth
- Reference photos of a genuine 2022 caseback on a second screen
How to inspect the hippocampus
- Wipe the caseback with the microfiber cloth so skin oil does not fill the engraving.
- Set the watch on a flat surface and place the lamp to one side at roughly 20 degrees.
- Tilt the caseback until light rakes across the medallion and the seahorse throws a visible shadow.
- Look at the dorsal fin and the small side fins first, since these thin lines collapse before anything else on a worn die.
- Follow the tail to the coil and count the visible spiral turns. A good cut keeps them separated; a poor one merges them into a lump.
- Check the wall thickness on both sides of the seahorse body. It should stay even along the full outline.
- Trace the wave border around the medallion and confirm the ridges repeat at the same depth.
- Read the text ring outside the border and compare letter weight against your reference photo.
- Confirm the serial and reference numbers sit in the correct position and use the same font weight as the rest of the ring.
- Take a macro photo at 45 degrees and zoom in. Decals and printed seahorses show a soft edge that bleeds into the field.
Red flags
- The seahorse has no shadow at any tilt angle.
- Fins and tail coils are missing or fused.
- The wave border varies in depth from one side to the other.
- The caseback sits proud of the case and shows a gap at the edge.
- The engraving dulls or rubs away within weeks, which points to a printed layer.
- Text ring lettering is heavier than the reference or spaced wrong.
Grading the result
A 2022 replica with a clean hippocampus, even border depth and correct text weight sits in the top tier. One with a readable seahorse but soft fins sits in the middle. One where the seahorse is a flat outline and the border fades is a budget build, and no amount of polishing will fix a shallow die. Judge this one detail before you judge the dial, the bezel action or the bracelet, because it is the hardest part of the watch to reproduce and the easiest part to inspect in under a minute.