After working his way through the submarine labyrinth of the island, the plucky fox happens across a dark, secluded cave hosting (for some reason) one of the invading Battle Kukku Army's most heavily armoured combat mechs. A bulbous purple battlesuit, the Golem waddles backwards and forwards, periodically lashing out at Tails with its extensible arm and dislodging rocks from the cave roof.
Simply dodge the attacks and batter the mech with bombs. As you do more damage, the faceplate blows off, exposing the bedraggled Kukku pilot. Further application of explosives destroys the machine and rewards Tails with the Anti-Air Missile for his Sea Fox submarine.
If you return to Cavern Island later in the game, you can still find the pile of scrap which the Mecha Golem leaves behind, remaining on the floor of the boss cave in perpetuity.
Continuing in their tradition of stationing powerful combat robots at the bottom of isolated cave systems where they have apparently no chance of doing anything useful, the Battle Kukkus have another Mecha Golem in place in Volcanic Tunnel - ostensibly the second level, but Tails needs the Great Bomb to access it, which is found in Polly Mountain 2.
More like a static gun emplacement, this Golem simply sits in position, strafing the area with Vulcan cannon bullets. Fly over his head and drop more great bombs on top of him; or, if you're feeling sneaky, take the long route to the boss chamber which brings Tails out behind the robot, able to lob Remote Bomb after Remote Bomb through a tiny fissure and thus take out the boss in complete safety.
As before, after enough damage has been done, the mech's faceplate blows off to reveal the pilot. Once destroyed, this machine's wreckage remains forever too. Fly through the now-open upper passage to pick up the Extra Armour upgrade for the Sea Fox.