And behold, it is wisdom that this land should be kept as yet from the knowledge of other nations; for behold, many nations would overrun the land, that there would be no place for an inheritance.
Wherefore, I, Lehi, have obtained a promise, that inasmuch as those whom the Lord God shall bring out of the land of Jerusalem shall keep his commandments, they shall prosper upon the face of this land; and they shall be kept from all other nations, that they may possess this land unto themselves. And if it so be that they shall keep his commandments they shall be blessed upon the face of this land, and there shall be none to molest them, nor to take away the land of their inheritance; and they shall dwell safely forever. 2 Nephi 1:8-9

With these verses, the Book of Mormon explains why no surrounding nations knew of these nations, that originally sailed from Israel around 600 BC.  God said that he would keep them secret to protect them.

However, there would be evidence revealed today of such a vast and powerful nation as described in the Book of Mormon.

• There would be archaeological evidence of these cities.  None have been found.
• There would be linguistic evidence with Hebrew influence showing up in Native American/South American language.  None has been discovered.
• There would be accurate biological references in the Book of Mormon to the plants and animals.  Little accuracy has been found.
• There would DNA evidence in the people today linking Middle Eastern DNA with Native American DNA.  None has been found.

While God could have kept a nation secret during its existence, after its existence we would be able to see evidence in many different areas.  Even those cannot be discovered.

Is God still keeping it a secret?

If the Book of Mormon is true, wouldn't God want people to know in the same way he reveals the truth of the Bible?