Is it true that all salt is sea salt?

In a very real sense, yes, that is a true statement. Virtually all sodium chloride on earth was formed in the ocean. It’s a bit of a misnomer to consider “ocean” to be synonymous with “sea”; technically, and according to the National Oceanic and Atmospheric Administration there is a difference:

Many people use the terms "ocean" and "sea" interchangeably when speaking about the ocean, but there is a difference between the two terms when speaking of geography (the study of the Earth's surface). Seas are smaller than oceans and are usually located where the land and ocean meet. Typically, seas are partially enclosed by land.

Since many manufacturers of “sea salt” evaporate ocean water independent of any actual “sea”, I’ll use that small misnomer too, and equate “sea” to “ocean”.

Salt (sodium chloride) deposits were born in ancient oceans in a process that started over four billion years ago. At one point in time or another, almost all of the planet (exactly how much and when is still a matter of debate) has been under an ocean. Rivers flow into the oceans, bringing with them minerals from erosion including positively charged sodium. Volcanic eruptions from the ocean floor spew gasses into the ocean including negatively charged chlorine.

Salt lakes share a quality with oceans in that they don’t have an outflow, the volume of water is balanced by evaporation and rainfall. Water evaporates, but salt does not. So the salt created (and is still being created, but at a much slower rate) by the oceans concentrates in an ongoing process in salt lakes.

Salt mines are just dried up oceans.

So, marketing sea salt as special because it’s from the sea is a bit disingenuous. There are many reasons to choose one type of salt over another, but that it comes from the sea isn’t one of them. Other questions here address trace minerals found in certain mined salt, the addition of iodine in some table salt, crystal size and refinement methods; but the original source of all salt is the same.

There is a kind of fun exception: Some salt on earth comes from asteroids and meteorites. Garnish with that, Thomas Keller.

Yes and no.

Sea Salt is salt that is harvested by evaporating natural brine - seawater, typically. Table salt is salt that is mined and processed for purity of color and flavor, and sometimes adulterated with nutrients like iodine and anti-clumping agents. The two will taste very different, as Sea Salt will have trace minerals and even biological elements that affect flavor. The flavor and even texture will often change from one part of the world to another and with harvesting techniques - Brittany Fleur de Sel is very different in taste and texture than the stuff in the paper canister of Morton's Sea Salt.

Salt itself is formed by geological processes that typically involve the oceans - mineral runoff from land will contain sodium ions and volcanoes on the seafloor will contain chlorine ions, and they find each other in the oceans to make sodium chloride. Once water in any given area of the ocean reaches saturation, the excess salt will precipitate out of the water into halide crystals, and these deposits of salt crystals are often vast, and will be buried by other geological processes (sediment accretion, volcanic activity, etc.) These crystals can have different flavor aspects as well - Himalayan salt apparently gets its delicacy and taste from iron deposits!

Halides will also form if seawater, or really any sort of salt in solution - is evaporated. This is how sea-salt is harvested.

Here is an excellent article on the geology of salt.

This is an straightforward overview of the differences between the various types of salt - mined vs. harvested, unprocessed sea-salt vs. kosher salt, etc. - from an online retailer of culinary salt varieties.

Technically, yes.

Consumers buy sodium chloride from three main sources (in order of increasing rarity/expense): underground mines, evaporation ponds and sea coasts.

• Underground mines were formed when salts concentrated in large bodies of water which evaporated over time. Many of these salt flats (think Great Salt Lake, Bonneville Salt Flats, etc.) wound up underground over centuries and are later mined to produce (usually table) salt. It's entirely possible that some of these are/were mined while the dried-up lake was still on the surface.

• Evaporation ponds are (usually man-made) ponds that let in salty water, then are closed to allow the sun to evaporate the water, leaving the salt behind to be harvested. Examples include the (former) salt ponds of San Francisco Bay, among others.

• The third common source is salt that is manually scraped off of rocks on the sea coasts of several areas.

There are probably also other low-level production salt sources.

What these all have in common is that the salt was at some point concentrated from large bodies of water that evaporated. The generic term for these large bodies of water being "seas".

Is my salt "Sea Salt"?

Today in the US, the term "Sea Salt" is most commonly a marketing term used to compete with the salts that are hand-scraped from coastal rocks.

By modifying the table salt production process, larger crystals reminiscent of hand-scraped salt can be produced.

Some manufacturers market these larger crystals under the marketing term "Sea Salt", first to indicate the larger crystal size (think Kosher salt) and secondly to capture the higher prices afforded to the hand-scraped (read: labor-intensive) "boutique" salts.

You could chemically create table salt by literally reacting chlorine and sodium (do not attempt to do so in your kitchen :), or by any number of anorganic reactions, or by burning organic matter and isolating the salt therein - but even here, the sodium and chlorine will have originally come from sea salt. And there would be no likely reason to use such an artificially created salt in food (laboratory uses with special purity requirements are a different matter), it would be more expensive to make.