If you haven't spotted one yourself yet, we've got some coordinates you can try.
I spent most of the day yesterday trying to track down No Man's Sky's sandworms, which were added in the Origins update. I was operating on the logic that a sandworm probably likes sand—seems sensible—so I visited desert planets, parched planets, arid planets—anything with a cactus and some dirt.
Turns out, based on observations from the community, No Man's Sky's sandworms will appear on a wide variety of planets, not just desert types, so seeing one may be a completely random event. Many players have reported seeing them on planets with some radioactivity or radioactive weather, and I finally found my first sandworm this morning on a planet with radioactive humidity. Others report seeing them on lush planets with peaceful weather. Some have spent hours searching, others have started playing and spotted one within minutes. It seems like a crapshoot, so why leave it up to luck?
How to find a No Man's Sky sandworm
Below you'll find the coordinates for two planets I've seen sandworms on. One of them even spawns a sandworm within a few seconds every time I visit.
The first set of coordinates listed below are for a moon, which was posted by Random-monkey on Reddit, and the second set is for a planet I found myself. If you haven't used coordinates in No Man's Sky before, I'll explain that a bit further down.
Using the coordinates, take a Portal to the planet, hop in your ship, take off and land somewhere, and hop out. Exiting your ship will create an autosave. Just load that save up again, and wait for a bit. When I'm on the moon listed in the top coordinates, a sandworm pops up within a few seconds of loading my save, every single time I try it. I even left the planet and returned, created a new save, and it still happens. It's worm city over there.
I don't have quite that kind of success with the lower coordinates, which is my own planet, but I've seen a sandworm there twice so it's definitely worth a try too.
So far I'm not sure what the purpose of the sandworms are. I shot one with my ship's cannons but didn't see any damage numbers, so they're probably not there to fight. They don't do any actual damage to the planet, either. They might just there for some cool and exciting sightseeing opportunities.
How to find and use a Portal in No Man's Sky
If you haven't used coordinates before, you'll need to enter them into a Portal. Portals are located on single every planet and moon in the game. Finding them isn't always easy (there's only a single Portal per planet, and planets are pretty big) but if you have an exocraft with an upgraded scanner, you can use it to pinpoint monoliths, and if you find a monolith, it can tell you the location of a Portal provided to you have a Gek Relic, a Korvac Casing, or a Vy'keen Dagger to trade. You can find those items in containers, buy them at trading posts, or receive them as quest rewards.
Failing that, you can also find Portals by using planetary charts purchased from a cartographer on a space station.
Once you've found the Portal, power it up (it takes a lot of minerals), and enter the glyphs in the order shown in the images above. For way, way more on finding and using Portals, this article has it covered.