In Diggaman, you explore caverns, picking up keys, gems and other useful items while investigating the disappearance of your brother, a geologist. It combines puzzle and dexterity elements with 'dungeon'-style gameplay in a fun but somewhat unforgiving way.
The game mainly makes use of built-in functionss of ZZT. One is slightly tuned: you dig using ammo, destroying a group of rocks (a breakable) directly in front to you. It's pretty linear, with some additional (hidden) paths and bonus-caves.
A mining camp is your base. It's a campfire and couple of tents and it has the function of a town in an RPG: it's where you return after completing a there's a market for gems and other collectables. It's also where you notice most of the progress in the game. Eventually, you reach the final mine, defeat your nemesis and save your brother.
The gameplay is challenging, a nice take on the dungeon-exploration genre. The font, the style and the gameplay work well together. There are secrets in the walls and other unexpected encounters in the mines. I also like how his task is implied by his name, like with Gem Hunter. And there's a boss fight.
Unfortunately, the game has a couple of flaws. Items are scarce, there's a lot of trial and error and the game gives very few directions. The caves and tunnels are well designed, but it's possible to get stuck in many ways. I couldn't get past one particular puzzle for a long time - it seemed to require impossible timing and I zapped my way out.
The game is pretty hard: both for good (you have to be careful and adaptive) and for bad (you wander around and get stuck). Still, the game is enjoyable, it's (visual) design stands out and has a very characteristic feel.