When the main male lead character explores the drain tunnel, not only is it much taller or far more wider a diameter than necessary for a pool this size, it terminates in a vertical vestibule or worker shaft with a ladder that goes back up to the pool deck. Most likely, a drain tunnel for such a pool would run horizontally/ laterally straight out 90° from the basin, and either be a large unified stream of outflowing water, or the tunnels or pipes more likely would branch out into several smaller pipes, and then distribute the outflow of water to the environment, into the surrounding ground, returning the water to the water table underground.
