The Watchtower detected an increase in boom tube activity in Bialya. A squad from the Team were sent in to investigate hot spots in the hope they could find out more about the alien bomb used to destroy Malina Island.
While the Reach forces were distracted by the attempts to rescue the Team, Deathstroke opened a portal to the Warworld's stasis chamber, sending through some taser bolas to take out the guards, before coming through himself to steal the crystal key.
Once the Team was rescued, and Nightwing had dismissed Arsenal from the Team, Asami opened a boom tube to take Arsenal, her friends, and herself back to their safehouse.
In the comics, boom tubes are used by the inhabitants of the worlds New Genesis and Apokolips as created in Jack Kirby's Forth World series of comic books and used extensively in other DC properties.
A boom tube is summoned into being by Apokolips/New Genesis technology, usually in the form of a "Motherbox" or hand-held "living computer" capable of determining the destination. The tube appears with a blast of energy and resounding "BOOM" and connects unlimited distances in Space/Time through a short corridor. The Tube disappears quickly and without a trace if it is not actively kept open.