Whenever a land an opponent controls enters, this ability triggers only if another land had already entered the battlefield under that opponent's control this turn. It doesn't matter if that other land is still on the battlefield, is still under that player's control, or is still a land.

If multiple lands an opponent controls enter at the same time, Tunnel Ignus's ability triggers that many times, even if no other lands had entered the battlefield under that player's control earlier in the turn. That's because each ability takes into consideration the other land(s) that entered at the same time as the one that caused it to trigger.