[Riley]

I’ve always assumed it was some kind of Soviet submarine, but I’ve looked for versions on line without ever having success.

I have looked too, it seems that everything is on the Internet- except that.

[PLMerite]

"I have looked too, it seems that everything is on the Internet- except that."

Me neither. I know I've seen pictures of caterpillar tracks on the sea floor but I can't find them now.

Although the Swedes seem to have made a welcome sign (see below) which they have lowered into the ocean:

Strangely this picture appeared several times, and I didn't use any of "those" search words.