HM merchants can embark on a special quest to obtain a special container that is nearly weightless, but fits a lot of things in it. Additionally, this container will store items between logins (but not reboots).
The container quest takes place on Bimmisaari. Somewhere on the planet is a secret entrance to the special area. Only merchants of sufficient level are able to find the entrance.
Be aware that your skills as a merchant will be tested and that you may need to train them before you will be allowed to attempt the quest.
Note: Everything about this quest is considered secret!