Team Rocket and their super-sneaky Secret Hideout

This place is secretly Team Rocket HQ! Gross! When you walk in front of the Persian statue, a Team Rocket Grunt will be alerted and will come and battle you. Defeat him, and then avoid the next Persian statue, going south instead until you find a scientist near a PC. Defeat him and then use the PC to turn off the security system. Now those Persian statues will be deactivated.

Make your way into the depths of the hideout. Defeat and talk to the various Rocket Grunts here to get the passwords for the door (SlowpokeTail and RaticateTail).

Continue on and you will reach Petrel, a Rocket Executive. Defeat him! Once you do, the Murkrow will follow you around, shouting the password in Petrel's voice. Take it with you to the locked door, and the Murkrow will SCREAM the password (I find it funny that it doesn't just say the password, it screams it.) The door will open.

You and Lance will fight Ariana. Once you defeat her, Team Rocket will clear out of its HQ. That means that you and Lance can defeat the Electrodes that are powering the evil radio antenna. Be sure to capture one.

With the radio tower out of commission, you can now go to the Mahogany Gym.