So, I was (re)searching for those 3d worlds, and found that apart from Second Life itself there is similar software, called OpenSimulator (but it requires another version of viewer in my case, I use Firestorm).

You definitely will not mistake this kind virtual world with reality any time soon (too many visual glitches, low complexity) ..but may be even such platform can be used for ... some experiments.

Three additional links:
https://vimeo.com/200991382 - 1 hour long 'machinema' style video interview, found while searching for Rosehaven place.
Blog, where i found term Hypergrid
Local user/admin of OpenSIm instance - DataPacRat

I think I also added myself to Optimalverse group.

A bit more on system requirements:
https://www.outworldz.com/Outworldz_installer/technical.htm

Fred Beckhusen said on 2017-05-08 20:40:54
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@Mally McGinnis Welcome to the club! Glad you were able to get it to boot right up. I also hope you haven't broken everything already! haha (You will ) The server is both the hardware it runs on and the Opensimulator software. The client can be on any machine, including the server. (Client + Server = Standalone) The server can hold a lot in 8 GB of RAM. I have seen some nicely peforming 8 GB laptops with three avatars and some NPC's. Once you see how little it uses, you will realize the Linden Lab is really screwing people over for a core and 1 GB of RAM. You will find that Opensim mainly eats a bit to a lot of of CPU ( depends upon scripts and avatars) and lots of RAM and requires almost no disk or network speed provided you keep avatar count down. The database does quickly get large, and never shrinks. But disk space is cheap. If you load a lot of OARS, it will grow to many gigabytes, but that's nothing to worry about other than backing it all up starts to take some time. I have loaded every OAR there is on mine, and it runs 150 SL equivalent regions 24X7, and it's only 32 GB. Prims as a figure of measure here is obsolete. 15,000 to 25,000 is not a lot of prims. Over 100K starts to get pretty big to the viewer. I can make a mesh sim with a hundred thousand triangles, and it can still be just a few prims. So prims is a bad thing to look at. Starting out, you should more about textures on your builds. You can make a massive setup if you control every texture. That's not always possible, but try to keep your textures as small as possible. It only takes a few hundred 1024 sized textures to kill a viewer. You do need to worry about RAM. An 8 GB machine is a minimum to start. Less than that you really need more RAM. It's by far the cheapest thing to add. Don't worry about disk speed, or CPU. It does like multiple cores, an I5 good enough for almost anything but me, and can run a lot of regions, given the RAM. The viewer is another matter, as you probably already know. You will probably find people talking online about MAXPRIMS settings - the number of allowed prims that shows in the the viewer. You can ignore that, as it is meaningless here, yet people think it means something because some grids with an economy module support it. It is set to 15,000 by default and even if you changed it, it just stops at 45,000 due to viewer bug. It is meaningless, though. This isn't SL, there is no enforced limit on prims, or regions, or even on region sizes. Go ahead, fly that airplane across a sim 50 times SL in size. You will first notice the viewer struggling to render the 2 GB image that is the land, but Opensim doesn't really care about that. I once let loose a bad rezzer and filled Virunga up with 120,000 prims. Got laggy, but ran fine. Didn't realize I had used it over in Dragon for another week, and it had over 500,000 prims in it. It got laggy and took up 25 GB of RAM. I saw that on the Task Manager while we were online in another sim that was getting laggy. Same machine has 100 NPCs and three people. There were far too many prims to delete, would have taken days. So I just looked thru my autobackups to one whose size was not growing and restored it. For comparison, I run several hundred SL equivalents in Win10 in roughly 16 GB of RAM in Use (the machine has 32 GB) . The largest of these Opensim instances is 9 or 10 of the Dreamworld free sims. Updates to a grid: This version is Standalone only - it is designed to run only on one machine, so it is not expandable beyond that one machine and one instance of it running. I figured that anyone that wanted to set up a grid could just install it themselves. But in a survey, grid mode was what everyone wanted. That hopefully will go in the next release. It is Yet Another Grid as your Standalone is not compatible with Grid mode so has to be started up again and loaded from IAR and OAR. It will be a while before any updates occur as it is stable and my rewrite for economy, grid mode, and the Bird Module is still a ways off. I may fork it into an 2.0 that has to be reloaded from scratch - it has only a full grid mode and is incompatible database-wise. ~Fred

https://blog.inf.ed.ac.uk/atate/2015/12/16/opensimulator-varregion-tests/
this one telling us 256x256 dimension for 'region' can be also measured in (virtual) meters, 1:1 ....

Normally Second Life and OpenSim have regions which are 256m X 256m in size. Regions can be placed alongside one another to make up larger land masses, as is done with the Virtual University of Edinburgh (Vue) regions which can involve 12 adjacent regions (see live served map).