The easiest and most useful cavern in the whole game, with an extremely linear path to reach the door. There are no items to collect here, but you can find 3 trading points, where you can get supplies by trading the diamonds you have obtained so far. Better if you have a good number of them.
You will start the cavern above an inclined plane and end up directly above the pillar to press. Proceed to the right. Along the way you will find cavemen wielding a club, but they shouldn't bother you. You will come to a wall that you can climb on. Climb up and enter the black window sideways to activate a teleporter. You will come to what looks like some kind of settlement, with a gleaming window. To reach it, simply go up to the second lowest building, right next to you and walk to the right. An exchange point will open in front of you, where you can buy 8 ladders at once. You will notice that the diamond icon on the bottom menu has started flashing. Select the diamond from the menu and press the B button to buy the 8 ladders in exchange for a diamond. Then walk against the wall to the right of the shop, to be teleported back to the black window below. If you want to buy other ladders you can also repeat the process.
Then proceed to the right and you will reach another similar wall. Repeat the above procedure to enter another exchange point. Here you can buy 8 dynamites in exchange for a diamond. You should do it at least once. Go back downstairs and continue to the right again, to find another climbing wall. Here you can access a third exchange point. In this you can buy 16 piles of stones, in exchange for a diamond. Do this once or more and go down again, continuing to the right. The exit door is right in front of you, but time will probably be up by now.
No problem: go back and just to the left of the first exchange point, let yourself fall into the small gap. Continue to the left and climb up to reach the pillar again. Press it, then go through the cave quickly to reach the exit door, killing the enemies along the way. You will easily be able to reach it with more than 40 seconds left, thus finding 2 diamonds in the next bonus room.
Note: of course, stopping to shop is not obligatory. If you want, you can also run through the entire cave right away and reach the exit door in no time, but it would definitely not be wise. This cave is in fact the main supply point in the game and you should take advantage of it. Your only other shopping option will be in Cavern 7. However, you can only buy dynamite there.