Tunnel Ignus — ruling 1

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.

Tunnel Ignus — ruling 2

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.

Tunnel Ignus : règles, fonctionnement et interactions (MTG)

Tunnel Ignus is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Whenever a land enters under an opponent's control, if that player had another land enter the battlefield under their control this turn, this creature deals 3 damage to that player.

How it works : Tunnel Ignus

Original English text — Wizards of the Coast / Scryfall

#1January 1, 2011· Wizards of the Coast
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.
No other card is named in this ruling.
#2January 1, 2011· Wizards of the Coast
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.
No other card is named in this ruling.