If you are zerg, and your opponent is zerg and you cant pass there main defence when you already have a good defence a ground strike force and a hive... Build one queen and parasite something of your opponents so you can see there base. Then build a nydus canal and build the exit right in the middle of their base :) Note: must keep away from defence and wont always work

CreepColony's Opinion: Hydralisks are probably the best units to transport. Also, what I like to do it send an overlord over to his base where all his workers are. He probably will not notice your color since he has tons of drones there. Find a nice spot in the middle of the workers and build the canal exit, and then when it's done, send mass hydras!