The Mine is the fifth purchasable building in the game, initially costing 10,000 cookies. Each mine produces 40 cookies per second by mining out cookie dough and chocolate chips. When fully upgraded, they each produce 1600 cookies per second. Cookie mines also unlock miner grandmas after 15 of them are purchased. Miner grandmas are necessary for the production of the bingo center, and the initiation of the grandmapocalypse.
You need at least 15 mines to purchase the "Miner Grandmas" upgrade.
The ultimadrill upgrade's flavour text, "Pierce the heavens, etc." is reference to the anime Gurren Lagann , where lines such as "Your drill is the drill that will pierce the heavens!" are said several times.
It would cost a total 91.93 quadrillion cookies to get the 200 mines necessary for the "Coreforge" upgrade, the last 20 mines representing 94% of that total cost.
Never Dig Down is a reference to "Minecraft" where digging straight down can lead to death by lava.