Yoma, Chapter 8, Mishnah 7. The previous mishnayos dealt with setting aside mitzvos in order to save a life, and our mishnah continues with this principle and applies it to a case filled with doubts.
"Mi shenaflah alav mapoles":
The case is a person upon whom a building collapsed, leaving him trapped beneath the rubble, and the question is whether one may desecrate Shabbos in order to rescue him. In this case the mishnah lists three doubts:
"Safek hu sham safek eino sham" - it is uncertain whether there is in fact a person beneath the rubble.
"Safek chai safek meis" - it is uncertain whether he is still alive.
"Safek nachri safek Yisrael" - it is uncertain whether the trapped person is a Jew or not a Jew.
Even when all of these doubts come together in the very same case - it is unknown whether there is a person there, unknown who he is, and unknown whether he is alive - the mishnah rules: "Mefakchin alav es hagal". The pile of rubble is cleared away from the person, even using tools and heavy equipment, in order to save his life on Shabbos.
"Metza'uhu chai - mefakchin alav":
If the trapped person is found alive, the clearing of the rubble continues. Here the mishnah teaches us a novel point: even if it is known that the person is going to die, and all that stands before us is 'chayei sha'ah' - meaning that removing him from the rubble will add only a brief amount of time to his life and he will soon die - Shabbos is desecrated for him even for this minimal amount of life.
But if he is found already dead, the clearing is not continued on Shabbos, and one must wait until after Shabbos to remove the body.
In summary: our mishnah teaches that saving a life overrides Shabbos even amid multiple doubts - a doubt whether there is a person there, a doubt whether he is alive, and a doubt whether he is a Jew - and the rubble is cleared even with tools. We further learned that the quantity of life being saved is not measured: even 'chayei sha'ah' justifies the desecration of Shabbos, whereas when the trapped person is found dead, the clearing is not continued until after Shabbos.