Crystal Caves

A 1991 side-scrolling Platform Game (or rather, a series thereof) by Apogee Software (now 3D Realms), Crystal Caves features the unlucky space trader Mylo Steamwitz as he searches mines of the Altairian system for crystals so that he can invest into several get-rich-quick schemes. On his way, of course, he runs into countless monsters, hazards, locked doors, power-ups and other standard platforming fare. The goal on each level is to collect all the crystals and get to the exit.

The game was actually released as three separate episodes, each one featuring new levels and some minor changes in graphics. The first one was made available for free as a teaser, while the next two were available for purchase ($30 for both). The game is still sold on the 3D Realms site (although this might change soon, considering the company went under).

Not to be confused with Crystal Castles.

This game provides examples of:

 * Collision Damage: Two enemies leave behind corpses that harm you on touch.
 * Explosive Decompression: There is one air generator on each level; if you shoot it, the vacuum rushes in and Mylo immediately explodes.
 * Floating Platforms: Which can be switched on and off.
 * Get Rich Quick Scheme
 * Gravity Screw: Some levels have low gravity, a couple have reversed gravity, and some levels include a timed pickup for temporary reversed gravity. The God Mode cheat code also allows you to reverse gravity whenever you want.
 * Hand Cannon: Your pistol fires rockets.
 * Hearts Are Health: Your Life Meter is made out of these.
 * Inexplicable Treasure Chests: Scattered around, can be opened for score if you find a key.
 * Invincibility Power Up: The very rare red mushroom.
 * Invisible Block
 * Jump Physics
 * No Damage Run: You get a reward after finishing a level if you pull it off.
 * Poison Mushroom: The green mushroom kills you instantly when you touch it.
 * Segmented Serpent: A green snake-like enemy.
 * Side View
 * Stalactite Spite
 * Treasure Is Bigger in Fiction: Crystals the size of Mylo!
 * Underground Level: All of them!
 * Unwinnable: Some levels require pre-planning or trial and error to get through without getting stuck. Thankfully, you can retry any number of times.
 * Wall Master: The purple dinosaur from the title screen.