Colón dijo: "Aunque tenga que buscarlos en las cárceles, los encontraré."
Why is the subjunctive for the verb tener used? Can't the indicative be used instead of the subjunctive?
Aunque can be followed either by subjunctive or indicative, with a subtle difference in meaning.
Basically, if you use indicative, the clause is in fact true and integral to the non-aunque clause (and you could basically replace with "a pesar de que", or "in spite of" in English).
If you use subjunctive, you are saying that it's less important, or perhaps even non necessarily the case. You can replace this with "Sea o no que..." or in English "even if".
It's not really a matter of 'doubt', but the dependent phrase expresses a hypothetical. You are not declaring that you are searching prisons.