The Undercity was an underground settlement on the planet Ba'ku that was created by the ancient Progenitors civilization. At the height of their society, they created the Undercity where they labored within the Institute that served as the centre of their scientific knowledge. From this site, they intended to genetically manipulate themselves to bypass evolution in order to make themselves the pinnacle of organic life. However, they inadvertently created the Xenophore Seed which ravaged their interstellar civilization and though the Progenitors managed to eliminate it - their species was spent and seemingly passed away. Among the survivors was Xa'tal who was kept in stasis until she could find a means to free herself of the disease.
In the year 2376, the United Federation of Planets discovered the ruins of the Undercity and established an archaeological dig in order to study it, which was led by Starfleet Captain Jean-Luc Picard. Among the crew that were assigned to the dig were Lieutenant commander Data and Ensign Sovok. During this time, Romulans under the command of Admiral Ratok attacked and stole a sample of the Xenophore organism. (TNG video game: Hidden Evil)