A low drumming sound had been skirting the lower limits of perceptibility 
ever since they had left the cavern where the refugees slumbered. Nell had 
first taken it for the beat of submarine currents on the walls of the tunnel, 
but as it grew stronger, she knew that it was the Drummers talking to
each other, convened in some central cavern sending messages out across their 
network. Realizing this, she felt a sense of urgency verging on panic that they 
find the central assembly, and for some time they ran through the perfectly 
bewildering three-dimensional maze, trying to locate the epicenter of the 
drumming.