Also be aware that differences in ground potential can be an issue: https://www.truecable.com/blogs/cable-academy/how-to-fix-a-ground-loop

That is also something you can avoid by going with fibre optics.

Use conduit. Include a pull line in case you want to add any cables later without digging it up.

And like everyone else said, fiber is probably better for the distance. You could either have media converters on either end, or better probably a couple switches with fiber ports.

If you’ve got line of sight, your other option is a pair of directional antennas. For example: https://mikrotik.com/product/wireless_wire

Watch out with shovels. I've cut my wire once, trying to plant a tree. My neighbours (who are my oarents) were not happy. Insurance companies recommend to dig 30 cm to a meter deep.

It might be overkill, but you could check with your local zoning office if there's any pipes or cables already running through your garden, so you don't accidentally snip any. Because that can become expensive real quick.

The ground deep under your garden/house is often not your property, but that of the state or a gas company or something. So there could be any number of pipes running down there. Maybe even old mineshafts.

100m is the maximim distance one can run RJ45 cable. Get some good weatherproof cable, dig a trench and put it in pipe, and you should be good.

Fibre optic is generally better for this use case, but Ethernet would work fine too. Be sure to use CMX rated cable, as it's rated for outdoor use (uses a more durable, UV-resistant jacket, and is suitable for direct burial).

Run it in conduit so you can easily replace it in the future if needed.

Wireless mesh are much better now, but about 15 years ago , I extended my home network to me detached garage by burying cable. It worked fine.

Should also check to make sure you don’t have any utilities buried in the path you want to trench. Many localities will have a number you can call and they will tell the various utility providers to flag any buried infrastructure they may have in the area.

100m is probably fine but you are at the limit of the spec. If you don’t want to deal with re-running the line if you do end up have transmission issues, fibre is better.

Probably worth double checking local regulations but most don’t care about low power lines like Ethernet. Definitely put it in a conduit though, to both protect it and make any future modifications easier. Should also cover it with a layer of different medium, such as sand, so you know when you are near it if you do need to dig it up again.

Yes I have such a cable. Run inside a pvc pipe, partially underground but mostly on the fence. Its about 80m, terminates into a 5 port switch to bring PoE to a few external cameras and a OrangePi zero running my pons sensors and automations.

Addendum: while 100m is the theorical maximum, I have successfully run much longer cables (up to 150m, uninterrupted cable) with p2p between two switches, and did work just fine, so... Go for it.

Also, while the standard recumends to avoid cascading switches, again I am proof that it isn't an issue to have multiple switches in cascade on the same network. Not old style hubs, of course. So adding switches in the middle would increase distance even more

If I'm not mistaken, 100 meters is the max for cat 5/6 cable without switches or other hardware. There are some good directional antenna options for a wireless solution. And yes, a protective pipe (pvc) would be required. No direct burial.