Descent without a leap into the unknown
Build a staircase one block wide with a passage height of at least 2 block. Leave more space above the steps if your head catches when climbing. Don't dig directly under your feet or above your head: there may be a cavity under the floor and gravel and lava above. Reference: Minecraft: Caves & Cliffs II.
Water in a bucket helps with falling and working with lava in the Overworld, but in Nether, regular pouring does not create a source. Keep blocks in the adjacent cell to block the sudden flow.
One return route
Mark the entrance with coordinates and place the chest in front of the deep descent. In the passage, always place torches on the right as you move inward: on the way back they will be on the left. At each fork, place a sign to the exit. This is your navigation system, not the torch mechanic.
Regular hostile mobs in modern versions require complete darkness, but individual mobs and generators have their own conditions. Rare torches do not make any cave guaranteed empty.
For the diamond mine, use a separate layer and path plan: Where to mine diamonds and how to lay out your mine.
Ore next to lava
Do not stand on the mining vein. Open its edge, make sure there is a floor and seal the holes, then mine from the far edge towards you. An object that has fallen into lava will not be returned by water.
If you hear a stream, stop and find the source through a side passage. If there is a gravel collapse, retreat to the already excavated area; Don't keep hitting the ceiling from falling blocks. Return to the last point of pickaxe durability: the way back may also require mining.
A backup attempt after death requires a separate procedure: Recovering your items after death without dying again.
