A life on the ocean wave, or at least under it, with the Blue Peter theme drumming away in the background. Persistently. All the time. Without cease. I was half expecting John Noakes to pop out from behind a rock toward Petra in a diving mask.
Starting off in a submarine in a cavern, the first thing you do is crash into a wall and drown. After a while, things get better; you get out of the cavern and crash into a rock and drown, and wait until you get to the hard bit. Things get easier with practice and eventually you get to find your way around quite well.
It looks like the caves go round in a circle so you find yourself back where you started, although I'm not sure since life gets pretty hectic after you've got through a few levels. As far as I could see there was no way of avoiding the enemy missiles. You can't shoot them down and they home-in on you faster than you can run away. Occasionally, you can get one to destroy itself on a rock, but it's more by luck than judgement.
Still, I enjoyed playing it and it says a lot that I'm prepared to go back and spend a few more hours annihilating myself until I get it right.