Every swimming pool needs a filtration system to keep the water clean. The water flows to the filtration system through two main drains at the lowest point in the pool. These are covered either with grates or with antivortex covers to divert the flow of water so a dangerous vortex won't be created. The water also flows to the filtration system through multiple skimmer drains all around the top of the pool. These drains suck water from the top of the pool, skimming any debris, such as suntan oil, leaves or hair off the surface.
In one of the more common drainage systems, the inlet passageway of the drain has a floating weir, a door that swings in and out, letting in a little bit of water at a time. This water is diverted through a strainer to catch the larger debris, then through the filtration system and back out into the pool through inlet valves called returns. Aside from its main inlet, the skimmer system also has an equalizer line that leads to a drain below surface level. If the water level ever drops lower than the main inlet, this line prevents the skimmer from sucking in air.
In addition to the main inlet, the skimmer system has a secondary equalizer line leading to a drain below the surface level. This line keeps the skimmer from drawing air into the pump system if the water level drops below the level of the main inlet.
Because every pool has multiple drains, if one drain is blocked water can be pumped through the other ones. This way the suction is never strong enough to actually draw a person to a drain and hold him there. Most pools also have vacuum ports for cleaning the pool. Pool vacuum cleaners, which are usually driven by the pool's main pump, attach to the ports.