Level Up Unhatched Pokemon
To do this glitch, you must first execute the Pomeg Berry Glitch, which is done by using a Pomeg Berry on a Pokemon with 1 or 2 HP so that its HP loops to 65,535 (displayed as ?35 in-game). Do so, however, with only one Pokemon and an Egg in the party, with the Egg in the top-leftmost slot of the party in the Pokemon menu. After executing the glitch, you will now be able to enter battles and use the Pokemon Egg as if it were a normal Pokemon. It will function as a normal Pokemon - you can view the stats it will have upon hatching, it can level-up by beating Pokemon, it can learn new moves, it can even evolve. This means that you can alter the Pokemon inside the Egg in otherwise-illegitimate ways. For example. Pikachu cannot normally know Thunder when hatching from an Egg at Level 5; through this glitch, Pikachu can. Similarly, you can hatch Eggs with fully-evolved Pokemon such as Typhlosion, Meganium, and so on, inside.
Pomeg Berry Glitch
For this glitch to work properly, you must have a Pomeg Berry and a Pokemon who has not had their HP lowered as much as possible already through Pomeg Berries. (If the latter is the case, the use of HP Ups on the Pokemon will suffice.) Get the Pokemon to a severely low level of HP (1 or 2 HP). Once you do so, in the field, use the Pomeg Berry on the Pokemon. This will in turn cause the data in their HP to cycle back to the top of the counter, giving the affected Pokemon 65,535 HP - over 90 times the otherwise possible maximum in the game.
Unlockable How to Unlock National Dex Beat the Elite Four once and go to the S.S Aqua ship in Olivine City to get the National dex.
Contributed By: raytan7585.
Trainer Card Stars
By completing certain tasks in the game, you get stars added to your Trainer Card. (Of special note are the Shiny Leaves, found only by speaking with your Pokemon in certain areas on very rare occasions, the locations being Nature-dependent.)
Unlockable How to Unlock Star Defeat the Elite Four & Champion Star Give Ethan/Lyra five Shiny Leaves Star Beat records in 10 Pokeathlon events Star Win 100 battles in a row in the Battle Tower Star Complete the National Pokedex (except for Mew, Celebi, Jirachi, Deoxys, Phione, Manaphy, Darkrai, Shaymin, and Arceus)
Unlockable Pokewalker Maps
There are over 20 Pokewalker maps currently available to anyone and everyone who owns them. The trick is obtaining Watts and getting the National Dex. Note that some of the following are event-only, so the Yellow Forest (for example), is unattainable right now.
Unlockable How to Unlock Beautiful Beach Obtain 200 Watts Beyond the Sea Obtain a foreign pokemon via use of the GTS Big Forest Obtain 40,000 Watts and own the National Dex Blue Lake Obtain 2,000 Watts Dim Cave Obtain 1,000 Watts Hoenn Field Obtain 5,000 Watts and own the National Dex Icy Mountain Rd. Obtain 30,000 Watts and own the National Dex Night Sky's Edge Trade a fateful-encounter Jirachi onto your HG or SS Noisy Forest Available from the start Quiet Cave Obtain 100,000 Watts and own the National Dex Rally Wifi Event from January 29th to February 28th, 2010 (Japan) Refreshing Field Available from the start Resort Obtain 80,000 Watts and own the National Dex Rugged Road Obtain 50 Watts Scary Cave Obtain 20,000 Watts and own the National Dex Sightseeing Wifi Event from November 14th (2009) to to January 11th (2010) in Japan and from April 17th to May 31st, 2010 in Korea Sinnoh Field Obtain 25,000 Watts and own the National Dex Stormy Beach Obtain 65,000 Watts and own the National Dex Suburban Area Obtain 500 Watts Town Outskirts Obtain 3,000 Watts Tree House Obtain 15,000 Watts and own the National Dex Volcano Path Obtain 10,000 Watts and own the National Dex Warm Beach Obtain 7,500 Watts and own the National Dex White Lake Obtain 50,000 Watts and own the National Dex Winner's Path Wifi Event from May 6th to June 25th, 2010 Yellow Forest Wifi Event from April 1st to May 5th, 2010
Contributed By: marioman1154.
Unlock 8 bit music
Once you obtain all 16 badges, talk to the music designer in Celadon, and you unlock the 8-bit originals from the original gold/silver games.
Contributed By: big_one12.
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More Pokemon HeartGold Version from GameSpot