Even though this “game” is almost an adult (18 years old), there is so many people who haven't heard about Geocaching.
It all started in May, 2nd 2000.
- WHAT IS GEOCACHING?
Simply, Geocaching is an outdoor game or activity which will provide you a lot of fun and it will help you to discover new places, cities, blocks or investigate more already known places. If you posses a smart phone, and there is a few people who doesn't posses any, or GPS device you can participate in this so much fun activity. There is no age limitations so everyone can enjoy.
"Geocaching is legal in every country except, of course, North Korea (where GPS and all other mobile devices are illegal to possess)"
In general, Geocaching is hide and seek game in which you are trying to find an waterproof containers called "Geocaches (or 'cache')" which contains an paper log book in which you need to sign your username to mark it as found. You can also make your own Geocache hide it where ever you want and provide it's coordinates and data on Geocaching app so other people can try to find it.
- HOW CAN I DO IT?
So, first thin you need to do is to download an app on your smart phone and make an account. After that you just go to Geocache map which will show you all the "caches" near your location. If you click on the nearest it will provide you more details about it. Usually caches are placed near some important location such as important statues, squares, buildings, murals or just places/things you need to see. Also they can be hidden everywhere, so there's no rule about hiding caches.
For example, if there Geocaching app says that there is an cache on main square in your town and you are near so you want to find it. When you click on the map to guide you directly to it, it will show you standard GPS map and guide where you need to go (GPS will not guide you on exact spot of the cache, but it will bi within 10ish meter radius). Since GPS is not so precise you need to see more information about the cache, for example if cache is magnetic (than it's obvious that container is attached on some metal surface), or what is the size of the cache, or what is the difficulty of the cache. There are also hints if you need it – small riddle or clear information which can be useful to provide you and easier way to find it.
When you find it try to not be so excited and obvious because maybe someone else is also trying to find it and he is near you so please act calm. Sign the log book with your user name and date then return the container cache) on the exact spot where you found it so other people can find it also.
If you have any problems with cache or location, there are comments about every cache so you can see more info, more photos or report that something is wrong with cache (container is damaged, paper log book is full...) so the owner can fix the problem or act with the situation.
Final step is to mark cache as "found" in you Geocache app and leave comment how did you like it, or if you didn't find it mark it as "Didn't find it" and provide more information why you didn't find it etc...
- GEOCACHES
The first geocache was placed by Dave Ulmer on May 3rd 2000 near Beavercreek forest, Oregon, Portland.
As I mention, geocaches are usually waterproof containers in all sizes from "nano" to XL and can contain just paper log book and maybe pen/pencil or they can contain things to exchange with the owner of the cache. It can contain all kind toys, coins, papers, badges, currency, buttons and even CD's... so you can leave your item in the container after you find it and take one from the container.
Of course, food, danger and illegal stuff are not allowed to put in the container.
You can find Geocaches literally everywhere, on and in the trees or bushes, on the traffic signs, between rocks, underneath the rocks, in the water, hanging, stick... literally every situation and every city or even a small villages or abandoned places.
- IS THERE ANY GEOCACHE IN MY HOMETOWN?
Of course there is, you wouldn't believe how many. Let's just quick check Geocache map of some random towns:
ZAGREB
LISBON
LONDON
NEW YORK
See? There are everywhere!
- MOST INTERESTING and CRAZIEST GEOCACHES
Earth's Roof - Mount Everest Peak
- on the top of the Mont Everest
Geocache Description:
"You must remember that climbing the Mount Everest is not for beginners. Even the ones that are used to still die in this mountain.
Only try for this climbing with the right equipment, the right partner, the right weather and the right mind."
THE LEAP
Geocache Description:
"Though The Leap provides no room for error,
the jump will likely prove to be less
difficult than actually getting to the area."
REGISTAN DESERT
- in the middle of Afganistan desert – Registan
EAST BAF
- in the middle of the war zone within mine field
Geocache Description:
"This cache is located on the East Side of Bagram Airfield, Afghanistan. Elev. 4980ft.
WARNING!! BE MINDFUL OF MARKED MINE FIELDS SURROUNDING THE AREA!
There is no need to leave the well beaten (man made/paved/concrete) paths."
You can also check here some of the interesting and cool picks:
https://www.buzzfeed.com/amandam10/20-ridiculously-creative-geocache-containers-1kzg?utm_term=.es25mDo5W#.gt8DdoYDA
MY EXPERIENCE?
I was searching caches in every town I visited, except my home town hahaha. I don't know why, perhaps I'm too lazy when I'm at home to go and seek caches, but I have logs in Valencia, Santiago de Compostela, A Coruna, Gijon, Lisbon, Almada...
Last ones were from Almada couple day ago. 
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When I heard about Geocaching it sounded really stupid for me, but when I tried it for the first time it was so exciting and fun, so I dare you not to judge but to try it and you will see...
Till then...
... Kind regards!
GG