Why is there a hole under the door?

its called a gap, not a hole..there has to be some clearance for the door to open and close. most doors have a gap to accommodate for carpeting

Because the door sweep has worn off, or was never installed. Doors are cut slightly smaller than the opening it fits into (door casing) so it will open and close without sticking. A rubber "sweep" is then attached to the bottom of the door to fill the gap between the bottom of the door and the threshold, so cold air can not enter the house.

Over time, the sweep wears off or gets brittle and disintegrates. When that happens, it leaves a gap at the bottom of the door that should not be there.

Door sweeps are inexpensive, usually around $5 in the USA, and anyone who can use a screw driver and hack saw can replace or install one. Just knock the pins out of the hinges so you have access to the bottom of the door. Remove the old sweep if one is there. Screw the new sweep into place. Use the hacksaw to trim the sweep off even with the edge of the door. Put the door back into the casing and install the hinge pins into the hinges. It takes less than 15 minutes total time.

what has this got to do with astronomy?

It is there so the landlord can slip you the eviction notice for not paying your rent. I strongly suggest you read that notice right away.

That is something known only to the ancients.

Philosophers have puzzled over it for centuries, in vain.

No definite answer exists today. Only speculation.

Cheers!

On an interior door there is a piece of trim molding (actually spelled moulding) around the top and the sides, sealing the gaps when the door is closed . The bottom of the door is cut short for carpet, tile or wood flooring, which is often installed after the door.. A door threshold can be installed to fill the gap.

There's a frame around 3 sides of a door, but you don't want an obstruction in the doorway, so there's no frame at the bottom on internal doors, and you need a gap so the door doesn't scrape on the floor.
External doors often do have a weather bar that you have to step over, that should stop draughts and water coming in under the door. If there's a big gap under any door it just hasn't been fitted very well. Except for toilet cubicle doors where it's good to have ventilation.

there is usually free clearance above the door (more wall up to the ceiling) so no chance of scraping the ceiling when opening and closing the door. However, at the bottom, if the floor is uneven or the door hung slightly off horizontal, it could scrape the floor on opening and closing, so it is usual to hang the door with some modest clearance at the base, just in case.

The gap = slot allows air to pass under the door for air conditioning and heating.

its not a hole ......its a gap .........depending on what the floor is finished in then the gap shall vary .........more for carpet than vynl

its not a hole ..its a gap ...depending on what the floor is finished in then the gap will vary ...more for carpet than vynl

Ok

That's cool Quadrillian is right

On exterior doors probably cos the weather bar has come loose and it's been thrown out with the trash by someone. The weather bar fits accross the door width to the sides of the bottom of the door frame on a timber door and has rubber seal on the long side or fitted to the base of the door that the door fits up against when closed to stop rain coming into your home.

See link for details.
http://www.diydoctor.org.uk/projects/weatherboard.htm

So the Mexicans can sneak into your house lol

To prevent from committing suicide.

It is not a hole. It is clearance space. It may be much more than is needed. You could install a sweep as Happiness suggests. I in fact had just such a gap and I did indeed install a sweep. This made a lot of sense because I heat/cool that room separately, even though it is an inside door. Now I am cozy. If it were an outside door then a sweep is required to keep the weather out. They also make a rolled rubber threshold piece which closes the gap from below.
In my porch the floor slopes but a brush type sweep works well to keep out the spiders and provides some draft resistance.

for you to breathe more

for bugs

I had some mice chew under my door for access to my house.

my testicles are itchy

For peeping toms.

It's for the trolls to crawl in and out

A gap

It's a space so the door can close over carpeting. It also allows for air exchange in the room when you fart heavily.

It is not a hole. It is clearance space. It may be much more than is needed. You could install a sweep as Happiness suggests. I in fact had just such a gap and I did indeed install a sweep. This made a lot of sense because I heat/cool that room separately, even though it is an inside door. Now I am cozy. If it were an outside door then a sweep is required to keep the weather out. They also make a rolled rubber threshold piece which closes the gap from below.
In my porch the floor slopes but a brush type sweep works well to keep out the spiders and provides some draft resistance.

For pets and parcels.

Not all dots have one. Any frame at the bottom makes something to trip over and is easily damaged.