You have no key, the cave is empty and you need to get out.
Well, the cave is not completely empty. Pick up the FLUID lying in front of you (1/1). Use the arrows at the top to travel forward to c 3400s. In the left corner is a pool of water. Use your fluid on the water (3/4) and travel forward in time again. Stop at the Age of Floods and see that in the left corner a crystal structure has grown.
Use the structure to take a piece of CRYSTAL with you (1/5). Travel back to the c 3400s and give the crystal to the lady (2/7) who’s very interested in pretty rocks. She’ll take off to show it at home and you can take the PICKAXE (1/8).
Travel back in time and go to c 1600. Talk to the old man and discover he’s in need of fresh air. Use the axe on the far left dark brown part of the wall. The old man will walk towards the hole you’ve created (3/11). Travel forwards in time to the c 2000 and see the skeleton near the wall. Use the skeleton to find the KEY (3/14).
Use the key on the door above the ramp (1/15) and when you walk through you’re free.
Game source: A copy of the game was found here on the internet.