Question: Why is it impossible to achieve a completely vertical position through entry when diving?
About the article "Diving", section "Entry".

Candidate passages:
The rules state that the body should be vertical, or nearly so, for entry.
Strictly speaking, it is physically impossible to achieve a literally vertical position throughout the entry as there will inevitably still be some rotational momentum while the body is entering the water.
Divers therefore attempt to create the illusion of being vertical, especially when performing rapidly rotating multiple somersault movements.
One technique is to allow the upper body to enter slightly short of vertical so that the continuing rotation leaves the final impression of the legs entering vertically.
Another is to use "entry save" movements of scooping the upper body underwater in the direction of rotation so as to counteract the rotation of the legs.
The arms must be beside the body for feet-first dives, which are typically competed only on the 1m springboard and only at fairly low levels of competition, and extended forwards in line for "head-first" dives, which are much more common competitively.
It used to be common for the hands to be interlocked with the fingers extended towards the water, but a different technique has become favoured during the last few decades.
Now the usual practice is for one hand to grasp the other with palms down to strike the water with a flat surface.
This creates a vacuum between the hands, arms and head which, with a vertical entry, will pull down and under any splash until deep enough to have minimal effect on the surface of the water (the so-called "rip entry").
Once a diver is completely under the water they may choose to roll or scoop in the same direction their dive was rotating to pull their legs into a more vertical position.
The magnitude of angular momentum remains constant throughout the dive, but since :angular momentum = rotational velocity × moment of inertia, and the moment of inertia is larger when the body has an increased radius, the speed of rotation may be increased by moving the body into a compact shape, and reduced by opening out into a straight position.
Since the tucked shape is the most compact, it gives the most control over rotational speed, and dives in this position are easier to perform.
Dives in the straight position are hardest, since there is almost no scope for altering the speed, so the angular momentum must be created at take-off with a very high degree of accuracy.
(A small amount of control is available by moving the position of the arms and by a slight hollowing of the back).
The opening of the body for the entry does not stop the rotation, but merely slows it down.
The vertical entry achieved by expert divers is largely an illusion created by starting the entry slightly short of vertical, so that the legs are vertical as they disappear beneath the surface.
A small amount of additional tuning is available by 'entry save' techniques, whereby underwater movements of the upper body and arms against the viscosity of the water affect the position of the legs. 
Night diving is a Underwater diving done during the hours of darkness.
It frequently refers specifically to recreational diving which takes place in darkness.
The diver can experience a different underwater environment at night, because many marine animals are nocturnal.
There are additional hazards when diving in darkness, such as torch or flashlight failure.
This can result in losing vertical visual references and being unable to control depth or buoyancy, being unable to read instruments such as dive computers and diving cylinder contents gauges, and potential separation from the rest of the diving group, boat, or shore cover.
Even with a functioning torch, these hazards are still present in night diving.
Backup lights are recommended.
Normal requirements for night diving are a dive light, and adequate protection from exposure.
Some precautions and skills for night diving include: avoiding shining your light in other divers' eyes, to be aware of and use surface light signals for bearings, or similarly use an illuminated shotline buoy.
A surface marker buoy with an attached strobe light or cyalume stick can be used to indicate the position of the divers to the surface team.
The use of a strobe light by a diver under water can damage the night vision of other divers. 
The aileron roll is commonly executed through the application of full aileron in one direction.
In some lower powered general aviation and aerobatic training aircraft, prior to applying aileron input, the pilot must begin the maneuver by trading altitude for airspeed (i.e.
diving).
This helps achieve enough airspeed to complete the roll without losing rudder and aileron control.
The minimum airspeed needed depends on the aircraft's design, but is generally about 120 to 200 knots.
Because full aileron is applied, structural limitations prevent many aircraft from performing the maneuver at very high speeds.
Starting from level flight, the pilot pitches the aircraft up about 10 to 30 degrees above the horizon, into a brief climb.
The purpose of pitch-up is twofold.
This causes an increase in altitude which minimizes altitude loss and airspeed gain.
As the aircraft begins to roll, it starts to lose lift.
When the wings are vertical, the only lift generated is a small amount from the fuselage, and the aircraft will begin to lose altitude.
The brief climb compensates for the loss, allowing the aircraft to complete the roll at the same altitude the maneuver began.
When the aircraft is completely inverted, the increased pitch results in greater angle of attack, enabling the inverted wing to generate lift.
After the initial pitch-up, the pilot places the elevators in the neutral position.
Failure to do this will cause the aircraft to continue pitching up during the upright part of the maneuver, and downward in the inverted part, resulting in something resembling a barrel roll.
The pilot then applies full aileron, accomplished by moving the stick to either the right or left.
As the aircraft rolls about its longitudinal axis, the nose will begin to drop.
Upon completing the roll, the nose will usually be 10 to 30 degrees below the horizon, so the pilot will need to pitch-up to return to level flight.
An aileron roll is an unbalanced maneuver.
As the roll begins, the aircraft will have a tendency to yaw away from the angle of bank, referred to as "adverse yaw."
The pilot will usually need to apply the rudder in the direction of the bank to keep the aircraft balanced.
An aircraft performing an aileron roll will actually fly along a slightly helical path, and a very light, positive g force will be maintained.
An aileron roll is similar to the slow roll, and the two maneuvers are often confused with each other.
However, unlike a slow roll, an aileron roll is performed at the maximum roll rate, and is uncontrolled in the pitch axis.
It consists of a constant attitude change during the maneuver; from the initial pitch-up to the plane following a slight corkscrew path as the nose drops, followed by the final pitch-up.
If the pilot picks a reference point on the horizon, directly ahead of the plane, the nose will actually appear to trace a shape similar to the letter "D" above this reference point. 
Penetration diving or overhead diving is a type of diving where the diver enters a space from which there is no direct, purely vertical ascent to the safety of breathable air of the atmosphere at the surface.
Several types of dive meet this condition: cave diving, wreck diving, ice diving or diving other man-made, underwater structures or enclosures.
The fact that the diver cannot immediately ascend to breathe from the atmosphere at the surface makes this type of diving much more risky than open-water diving.
It is essential that divers doing penetration dives on scuba are appropriately equipped and trained in technical diving techniques such as:  using redundant breathing gas sources  using distance lines to guide the diver to the exit When the dive depth and time are extended, the use of breathing gases other than air can reduce the risks.
Nitrox is used for relatively shallow diving and accelerated decompression.
Breathing gases containing helium such as trimix are used for deeper dives to reduce the enervating effects of nitrogen narcosis.