Up to 4 million passenger journeys take place on the tube every day. Well, around 4 million human passenger journeys. There's also a few thousand dogs, the occasional cat in a carrier, birds, ferrets...and mice. So many mice. It is currently estimated that there are half a million mice living and travelling around in the London Underground system. At present, it's not unusual to catch a glimpse of them running across the tracks while you wait for your train but experts suggest they may be in for a rough future. With the introduction of the night tube, several lines which had formerly been safe sleep-zones for the rodents will now be running all night, which will disrupt their patterns significantly.