It has occured to me that you might not have the str or materials to get to the level below. The only other option would be prospecting above ground or just mining in random directions until you hit ore.
There are some techniqqqques you can use to make mining easier, one method to note is called snake mining.

Above is the sekrit techniques, each black square is one tile in game, the arrows are the way your character MUST be facing while mining. The white squares are un mined tiles. Apologies for the rushed image
