DIGGER (aka. Monster Digger, 16K ZX81)
by Michael Pike
from Your Computer (December 1984), pages 155-157

Michael Pike traps monsters in holes on his ZX-81 with 16K expansion.

Your are the cursor at the bottom of the screen and you have to run up and
down the ladders avoiding the mutants, digging holes and dropping the
mutants through them before your oxygen runs out. You are given a fresh
supply of oxygen each screen and the number of mutants gradually increases
as the screens are completed to a maximum of eight. A full list of the keys
used for movement is given in the BASIC program before the game starts.

You can dig holes on any level except the bottom line and you are not
allowed to dig two holes right by each other or dig next to a ladder. You
cannot dig or fill while on a ladder as you will only move up or down it.
To dig you press one of the down keys and a hole will appear next to your
right or left depending on which way the cursor is pointing. Holes can be
filled in using one of the up keys but will not work if there is a mutant
in the hole.

To push a mutant through a hole move over it and jump using an up key. If
you wait too long the mutant will start to flash showing you that it is
about to pull itself up and mutate further into a stronger life form. There
are three types of mutants of increasing strength  S, $ and inverse $,
points 50, 100, 200. A $ and inverse $ have to be dropped through 2/3 holes
to kill them straight away and to obtain the full points or you can drop
them through a lesser number of levels to weaken them but you do not get as
many points. Also you can drop through the holes to escape the mutants.

If you master the game too quickly there are two POKEs you can enter to
make the game harder.

POKE 18750 will cause the oxygen to decrease at a different rate  it is
presently set at 3. Poking with 0 will make the oxygen stay at 6000.

POKE 18899 will change the speed of the game  this is initially set to 6.