It certainly look unusual to me.
I'd be interested in seeing it myself.
Is it a Java world?
Do you have seed?
Modded or un modded?
Coordinates of the image.?
Just because you find something in a world that seems inexplicable doesn't mean that Herobrine caused it.
As a matter of fact since Herobrine has NEVER existed in any vanilla version of Minecraft that's not even on the possible causes.
As the OP found out it was just caused by a lava flow.
Note: the configurations of lava flows are affected to some extent on when and from what direction you approach it.
I used to have problems keeping wooden gates on a stone fence I had built until someone pointed out that my 2x2 lava filled garbage pit was too close to the gate and at times set fire to it.
I moved the pit and my gates quit disappearing.
Since this thread was started on June 29, 2012.
My best guess for the Java release used is 1.2.5.
However it seems to work the same up through release 1.6.4 as far as the map goes but items in the Blacksmith's chest are different.
Below is the Amidst map of "8549017787595201279" in 1.2.5
Spawn is at 232, 256
"-3678191285988515738" spawns you next to a Snowy Taiga with a village north of you in a Snowy Tundra.
That's the only seed I found that is even close to your request.
I suggest that you download Amidst v4.6, run it, select a VANILLA profile for the PCMC version you prefer,
and start hitting Ctrl-R. (the link is in my signature)
That will cause Amidst to display the biome maps of random seeds.
Ctrl-H in Amidst gives you the seed input dialog.
Amidst 4.6 works with MC Java releases 1.0 up to and including 1.16.5.
Hint: Place Amidst in it's own folder and create an empty file in that folder named "history.txt".
When Amidst starts up it will detect that file and from then on will log of every seed that it displays to the end of that file.
That way, if you hit Ctrl-R too quickly you can find out what the seed was of the previous map and redisplay it.
I use Notepad++ as my default text editor as it will update itself if Amidst adds a line to history.txt while it is open in a Notepad++ tab.
Performance note: If you are looking for a specific biome at spawn for a 1.13.2 and above world, I suggest that you run Amidst with a VANILLA 1.12.2 profile to search (the world maps will be displayed about 3 times faster than with later releases). Once you find a world that seems to fit your needs hit Ctrl-C to copy the seed to your clipboard and switch profiles to a VANILLA copy of your desired release and use Ctrl-N Ctrl-V to enter that seed back into Amidst and display the desired map.
BTW: It is possible to have multiple instances of Amidst running at the same time. I currently have one for 1.12.2 and another for 1.13.2, 1.14.4, or 1.15.2 running.
That way I can search random seeds quickly with 1.12.2 and if I see an interesting one copy and paste the seed into the much slower 1.13.2 / 1.14.4/ 1.15.2 instance.
Please define "old world".
I solved the mystery.
The seed IS "-6062753212142760652" in Java.
I used release 1.16.5.
HOWEVER the world type is AMPLIFED.
Which means, IIRC, everything is 4 times taller.
The giveaway was the y axis value of the mushrooms base.
Here are a few screenshots of the mushroom and surrounds.
Here's two more views of the "island".
If you have a diamond pick axe there's enough obsidian available to reconstruct the portal.
There is more unusual things about this seed.
1. Spawn is on a ONE tree island. This has been a very rare occurrence for quite some time.
2. The one tree is a short oak that yields 4 logs and, for me, 4 saplings.
3. There are no flowers on the island BUT there is a BEE NEST sitting on the ground beside the tree trunk.
The bees disappeared almost immediately.
4. Because the Amidst map showed spawn on a small island I created the world in Creative with a bonus chest.
I suggest you do the same. Otherwise you have a really difficult survival island.
The attached image shows the hive and one of the two bees I saw.
Not having a Silk Touch tool I left the Bees Nest alone.
I created this world twice with bonus chests and the contents of the chest was different each time.
The second image shows the contents of the second try.
"-2397601474324577736" Fantastic!!!! THANK YOU for getting back to us.
I was correct. Almost all randomly created seeds are 19 digits long.
Here's the Amidst map for "-2397601474324577736" with the mine in question located.
Was level.dat_old corrupted too?
No, I see that all the time when I hit F3.
Around Java release 1.3.1 the server software that was a separate download for LAN multiplayer was integrated into the Java code so since then Java automatically has the option to "Open to Lan".