In endless mode when you send out your scouts sometimes a location entitled "Buried Dreadnought" will be discovered. When you send your scouts to this location your scouts will discover a buried treasure. When your scouts bring back the first treasure a townsperson will suggest building an archive to store any treasures the scouts find. This will unlock the building "Archives" in the construction menu. The Archives take quite a bit of resources to build, I can't remember the wood amount, but the iron is 500. Once you have built the archives when you click on the building it will show the treasures that you have found. You need to find 15 to fill the archives.
I got this achievement while going for the Medium Endless Endurance achievement as well. To get that achievement you need to survive 100 days. I got this achievement on the 106th day. I'm not sure what it is like on Serene Endless but on Endurance after 4 or 5 days of exploring you have to bring your scouts back so that they don't die in the storm. Because of this I started sending 2 scouts in the same direction, that way when there was 2 locations to scout I could send them to both, the Buried Dreadnought is usually at the 3rd or 4th location in a chain (although once I got it on the 2nd chain) so sending them in the same direction is helpful. This is Especially true if you find survivors or a core and you need that in your town, then you can send the scout group that finds these back to town and let the other keep searching for the Buried Dreadnought.
Once you land on the Dreadnought in Endurance mode, you are given a choice to retrieve it safely or risk your scouts lives. You must spend resources to get it safely. Sometimes it requires a steam core which are very valuable in this mode. Or you can risk it and it is a 50/50 chance that your scouts will die. In order to retrieve the treasure without risking the scouts or spending the core simply save when you get notice that they reached the Dreadnought. Then risk the scouts and if they die, load your save,and risk them again. Almost everytime they retrieved the treasure safely on the 2nd try, and always by the third.
Again, this may be easier on serene but it is totally doable on Medium Endurance and gives you something to shoot for the last 30 days when your city is pretty much self sustaining.
Once you get the 15th treasure back to the city the achievement will pop instantly. Hope this is helpful. Thanks.