Question: You often *plunge* into the sea or *plunge* you hands into your pockets.

Can the word be used to describe a "horizontal immersion"?

Example sentence:

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> She opened the door and **plunged** into the blinding sun.
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> 
> 

If not, what's a more appropriate word?

Answer: 
It's perfectly acceptable, but you might want to consider the "literal baggage," for lack of a better word, that the figurative term carries with it.

Take the OED literal definition that this derives from:

> 
> To thrust, throw, or drop into or in a liquid, penetrable substance, deep pit, container, etc.; to immerse, to submerge.
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> 
> 

In any case that you use the word "plunge," readers will associate it with the literal meaning to derive its figurative meaning. If you want to invoke feelings of falling deep into something, then it's appropriate. Just be aware that the use **ordinarily** refers to falling or pushing *in a downward direction*.
Almost all definitions of Plunging seem to imply that it's a word describing vertical descent.
[Merriam Webster](https://www.merriam-webster.com/thesaurus/plunge) thesaurus includes the following words as related:

> 
> dip, immersion, submersion; fall, plump, slip, spill, stumble, tumble; descent, drop; belly flop, header, jackknife, swan dive
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None of them indicating a horizontal direction.

Also, [Thesaurus](http://www.thesaurus.com/browse/plunging) synonyms imply a vertical nature.

It may be acceptable to use *plunging* in the sentence but you may not get the intended effect on a reader.
Maybe the following will suit better

> 
> She opened the door and **stepped** into the blinding sun.
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or

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> She opened the door and **charged** into the blinding sun.
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