Dig Straight Down is a minigame in Hermit Land. It was created by Grian in his Season 6: Episode 78. The idea of the game was that Hermits would dig straight down to try and find diamonds. At the bottoms of some of the places where Hermits would dig straight down, there was lava and if a Hermit fell in the lava, they lost the game.
DSD has 16 holes filled with stone and ores that can be found in the overworld. You use an Iron Pickaxe with Fortune III to dig straight down from the hole. You may bail out at anytime with 2 stacks of dirt you bring with you. There is a 100% chance of lava in the hole. It may have ores and resources in it. If you die, you lose all the resources you mined.