Starship Not-Enterprise exits hyperspace and looks at the planet filling up the window. Their jaws drop. The planet has a perfect geometric shape clearly visible from space, occupying the majority of the visible surface from their current vantage point.
Its astounding. Sharp boundaries, straight lines, perfect sharp corners, exact angles. Obviously this was created by a very intelligent alien life with large engineering capability and was perfectly planned. It looks like it was drawn in a CAD program.
However there is no mighty intelligent life on the planet and never was. There may be plant and animal life if it helps explain the creation of the shape. This perfect geometry was entirely created by natural processes. Somehow a natural process created something which which looks undisputedly artificial after one first glance.
If this is possible - how could a natural process can create a near-perfect geometric shape that's the most prominent feature on the planet?
Or, if not possible in the real world (which I suspect)…
What's the minimum handwaving one would need to do to get any near-perfect shape becoming the prominent feature on the planet by natural means?
The exact shape doesn't really matter - so long as a natural process can create it and it looks deceptively like an artificial process created it. Can be a Triangle. a Square. Letters or other simple glyphs. Any simple 2D shape. The exact design doesn't matter. It just needs to stand out as clearly artificial by way of being simple 2D geometry.
Am open to a curving shape, but a circle made by a conical volcano isn't going to cut it, the curve is unlikely to be a perfect circle, and the contrast against the surrounding land is going to be low.
With regards to projection any suitable map projection is fine. Straight lines on a sphere's surface bend in 3D space and can bend on maps, so this is open. It can be in true 3D space (eg 3 identical mountains with a perfect plane between their peaks that when viewed in 3D is a perfect equilateral triangle), or perspective to a point in space (that the not-enterprise happened to luckily intersect) so it looks perfect to the naked eye, or becomes a perfect shape when projected onto a standard, existing map projection, eg Mercator.
I have no idea of the best process to achieve this. It could be tectonics. Clouds. Erosion. Animal migration. Rainfall patterns. Shadows from other planets accumulating over time killing vegitation. Anything. That's why I'm asking. It can be transient but needs to be visible for at least a few weeks.
(If "perfect geometry" is considered opinionated for some reason. Any 2D shape from this list appearing prominently on a planet when viewed from space. Best answer is accuracy of shape, then simplicity of geometry.)
Edit: Changed "Perfect" to "Near perfect" - (a valid) quibble about perfection.