Pokémon FireRed and LeafGreen/Navel Rock

'''Navel Rock is an uncharted island between Floe Island and Chrono Island. Its topographical location is a closely guarded secret, as it is said to be home to a duo of legendary Pokémon who watch over the seas and the skies: Ho-oh and Lugia. Should this information fall into the hands of a power-coveting madman, it would spell disaster. Those deemed worthy to visit Navel Rock are presented with the Mystic Ticket, which will allow them access to this mysterious locale.'''

In order to access Navel Rock, you will need a Mystic Ticket, which is only available through Mystery Gift, or a Nintendo-sanctioned event. Unfortunately, this method is no longer viable due to the discontinuation of Mystic Ticket distribution. However, if you have a friend lucky enough to brandish the ticket, use the Mystery Gift to receive the ticket as a gift from them to access the island.

Option 1: Warp to Navel Rock
Should all else fail, there are underhanded ways of getting to Navel Rock- namely, Gameshark codes. If you own a Gameshark or have a Visual Boy Advance emulator with a patch, input the following codes depending on your game version:

'Fire Red: Warp to Navel Rock Automatically' 82031DBC 2402

'Leaf Green: Warp to Navel Rock Automatically' 6AADAC54 AQA4B6C7

Once you have the codes punched in, step through the nearest door or cave entrance you see. You'll be taken to the port for Navel Rock instead. Disable the warping code as to not start a repetitive loop appearing at the port. Instead, you'll be able to access the entrance to Navel Rock after walking out from the port.

Option 2: Buy a Ticket
The second option is a touch more convoluted, but it isn't as cheap as warping straight to Navel Rock.

Navel Rock
After you enter the cave portion of Navel Rock, it's simply of matter of moving forward to find the legendary Pokémon that dwell here. No wild Pokémon will pester you as you make your way onward, down one ladder to find a straight and narrow pathway. There are two small sections that deviate from this main path, but they simply lead to dead ends. They were rumored to contain the TMs Sacred Fire and Aeroblast, the signature moves of Ho-oh and Lugia, prior to the final stages of the game, but no such luck awaits you here- after all, making the moves of legendary Pokémon TMs would be too generous and cheap. Instead, keep going down the center path, where you'll eventually reach a wider section of the floor that has two ladders leading opposite directions: up and down. The descending pathway will take you to a grotto where a Level 70 Lugia dwells, while the ascending pathway will bring you to the summit of Navel Rock, where a Level Ho-oh will land before you.