...an entire world...

Great visual midrash from @thelucidreams:

Lucid Dreams - Sanhedrin.jpg

On Sanhedrin 4:5:

לְלַמֶּדְךָ, שֶׁכָּל הַמְאַבֵּד נֶפֶשׁ אַחַת, מַעֲלֶה עָלָיו הַכָּתוּב כְּאִלּוּ אִבֵּד עוֹלָם מָלֵא. וְכָל הַמְקַיֵּם נֶפֶשׁ אַחַת, מַעֲלֶה עָלָיו הַכָּתוּב כְּאִלּוּ קִיֵּם עוֹלָם מָלֵא

...to teach you that whoever destroys a single life is considered by the Torah to have destroyed an entire world; and whoever saves a single life is considered by the Torah to have saved an entire world...