Question: How can you breathe using a snorkel while submerged under water?
About the article "Snorkeling", section "Flooding and clearing".

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Freshwater apple snails in the genera "Pomacea" and "Pila" have an extensible siphon made from a flap of the left mantle cavity.
They use this siphon in order to breathe air while they are submerged in water which has a low oxygen content so they cannot effectively use their gill.
Apple snails use the siphon in a way that is reminiscent of a human swimmer using a snorkel, except that the apple snail's siphon can be retracted completely, or extended to various lengths as needed.
For these freshwater snails, the siphon is an anti-predator adaptation.
It reduces their vulnerability to being attacked and eaten by birds because it enables the apple snails to breathe without having to come all the way up to the surface, where they are easily visible to predators.
The shells of these freshwater snails have simple round apertures; there is no special notch for the siphon. 
Rat-tailed maggots are the larvae of the drone fly, "Eristalis tenax".
A characteristic feature of rat-tailed maggot is a tube-like, three-segmented, telescoping breathing siphon located at its posterior end.
This acts like a snorkel, allowing the larva to breathe air while submerged.
The siphon is usually about as long again as the maggot's body (20 mm when mature), but can be extended as long as 150 mm.
This organ gives the larva its common name.
It lives in stagnant, oxygen-deprived water, with a high organic content.
It is fairly tolerant of pollution and can live in sewage lagoons and cesspools.  
In ancient Roman and Greek times, etc., there were instances of men swimming or diving for combat, sometimes using a hollow plant stem or a long bone as a snorkel.
Diving with snorkel is mentioned by Aristotle (4th century BC).“Just then as divers are sometimes provided with instruments for respiration, through which they can draw air from above the water, and thus may remain for a long time under the sea, so also have elephants been furnished by nature with their lengthened nostril, and, whenever they have to traverse the water, they lift this up above the surface and breathe through it.“</ref> The earliest descriptions of frogmen in war are found in Thukydides' history of the Peloponnesian War.
The first instance was in 425 BC, when the Athenian fleet besieged the Spartans on the small island of Sphacteria.
The Spartans managed to get supplies from the mainland by underwater swimmers towing submerged sacks with supplies.
In another incident of the same war, in 415 BC, the Athenians used combat divers in the port of Syracuse, Sicily.
The Syracuseans had planted vertical wooden poles in the bottom around their port, to prevent the Athenian triremes from entering.
The poles were submerged, not visible above the sea level.
The Athenians used various means to cut these obstacles, including divers with saws.
It is believed that the underwater sawing required snorkels for breathing and diving weights to keep the divers stable.N.
Pierros is a Civil Engineer and author of historical essays.</ref>Italy started World War II with a commando frogman force already trained.
Britain, Germany, the United States, and the Soviet Union started commando frogman forces during World War II. 
Standards are offered for recreational diver training for the following grades of snorkel and scuba divers.
One Star Snorkel Diver - “a snorkel diver who is competent in the safe and correct use of relevant snorkel diving equipment used in a swimming pool or sheltered water.
The snorkel diver is familiar with relevant personal equipment and its use in a sheltered open water area.
The snorkel diver is ready to gain further open water training.”  Two Star Snorkel Diver - “a snorkel diver who has gained some open water diving experience.
The snorkel diver is considered ready to take part in dives with other snorkel divers, under supervision if a minor.
The CMAS 2 star snorkel diver is considered trained.”  Three Star Snorkel Diver - “A fully trained snorkel diver who has gained considerable experience in open water snorkel diving under various conditions.
The 3 star snorkel diver has acquired life saving skills and can lead snorkel divers in open water dives.”  Introductory SCUBA Experience - “this training programme aims at providing interested persons with an introductory SCUBA diving experience, to a maximum depth of ten (10) metres under the direct supervision of a CMAS Instructor, whilst using air as a breathing gas, in a safe manner.”  One Star Diver - “a diver who is competent in the safe and correct use of all appropriate open water scuba diving equipment in a sheltered water training area and is ready to gain open water diving experience in the company of an experienced diver.”  Two Star Diver - “a diver who has gained some open water diving experience and is considered ready to take part in dives partnered by a diver of at least the same or a higher grade.
The two star diver may dive with a One Star Diver in sheltered shallow water.”  Three Star Diver - “a fully trained, experienced, senior diver who is considered competent to supervise other divers of any grade in open water and support an instructor in pool and open water training.”  Four Star Diver - “a three star diver who has attained a higher than average level of knowledge and ability supported by broad diving experience.
He be able to assist in the training of One Star Divers and be competent to lead divers in order to accomplish major diving tasks or project objectives.”  
A swimmer's snorkel is a tube typically about long and with an inside diameter of between , usually L- or J-shaped and fitted with a mouthpiece at the lower end, and constructed of rubber or plastic.
It is used for breathing air from above the water surface when the wearer's mouth and nose are submerged.
The snorkel usually has a piece of rubber that attaches the snorkel to the outside of the strap of the diving mask.
An older technique is pushing the snorkel between the mask-strap and the head, but this practice increases the chances the mask will leak.
The optimum design length of the snorkel tube is at most 40 centimeters (about 16 inches).
A longer tube would not allow breathing when snorkelling deeper, since it would place the lungs in deeper water where the surrounding water pressure is higher.
The lungs would then be unable to inflate when the snorkeler inhales, because the muscles that expand the lungs are not strong enough to operate against the higher pressure.
Snorkels also constitute respiratory dead space.
When the user takes in a fresh breath, some of the previously exhaled air which remains in the snorkel is recycled into the lungs, reducing breathing efficiency and causing a build up of carbon dioxide in the blood, which can result in hypercapnia.
The greater the volume of the tube, and the smaller the tidal volume of breathing, the more this problem is exacerbated.
A smaller diameter tube reduces the dead volume, but also increases resistance to airflow and so increases the work of breathing.
Occasional exhalation through the nose while snorkeling will reduce the build up of carbon dioxide, and help in keeping the mask clear of water.