The comma after tunnel is not just any old comma; it creates the dreaded comma splice.
The problem is known by this rather arcane moniker because the comma splices, or joins, two clauses that it really should not be connecting. The sentence needs a pause that has more oomph. A semicolon would do, a period would do too.
There you go again. The sentence before last is another example of the same problem. It should read:
A semicolon would do; a period would do too.
One other thing: Should there be a comma after do? Nah. Really doesn't need it.
1. I see a light at the end of the tunnel; it's probably a mirror.
2. I see a light at the end of the tunnel. It's probably a mirror.
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