Of course, the short answer is that the Assyrians kicked them into oblivion (Damn you, Sargan II!) But we have to wonder (or at least, a bunch of wingnuts have to wonder) how that could happen? Why would our oh-so-protective god be watching out for us in Babylon and in Poland, but not be keeping an eye on those other 10 tribes during their Assyrian expulsion/captivity.
Nova has this to offer:
Over 2,700 years ago, the Assyrians exiled the ten tribes of the Kingdom of Israel. The ten tribes would have returned at once to the Holy Land had not the Lord encircled them with the legendary river, the Sambatyon. All week long, every week, the great and terrible river Sambatyon seethes with wild rapids, churning great rocks in billows up to the heavens. On the Sabbath, however, the river rests from its fury. But the ten tribes cannot cross because of their great piety and their reverence for the day of rest.
Which is myth, of course. But this site has done a great job of compiling what faint history there is. Pointing out moments when a tribe has claimed to surface… totally worth reading!
Beyond that, there are a lot of wacky sites on the web with explanations for where the lost tribes landed. Some far-out Christians LOVE the tribes. And people have had a lot of fun trying to track them down. Some of the claims aren’t so crazy, and maybe the tribe of Ephraim did land in India. Maybe the Ethiopian Jews are descended from Dan. All but Simeon. Nobody wants to be from the tribe of Simeon. Poor Simeon.
But be careful in reading on this subject… some of the wingnuts are more than nutty, and you should be on guard. “Lost Tribe” literature runs closely alongside conspiracy theory literature, evangelical literature, and anti-Semitic literature. This site claims that the antichrist will come from the tribe of Dan. They include a disclaimer:
To suggest that the Antichrist will be from one of the tribes of Israel is likely to incur accusations of "anti-Semitism" from those who would like to conceal this fact. However, we believe that the Biblical admonition to bless the descendants of Abraham includes exposing the identity of the man of sin who will lead many Jews to their destruction.
Yeah, that makes me feel better.
Finally… the most famous argument comes from the Mormons, who seem to think that native Americans might be the remnants of the lost tribes. Which (I guess) would make a little more sense out of their religion. People are even using DNA evidence to try and prove this stuff…