Sometime between the events of Half-Life and Half-Life 2, more specifically after the death of the Nihilanth and before the Combine invasion, earth was struck with portal storms, wherein the powers of the Nihilanth over the portals between Xen and Earth dwindled and fluctuated out of control. This caused Xen wildlife to uncontrollably warp down all over earth. When the Combine, who have some similar psionic, arrived at Earth through the portals, the storms managed to stabilize. The player never sees these storms themselves until the end of Half-Life 2 Episode 1, when escaping from City 17 on a train as the core of the Combine Citadel (The Combine's Earth-based Capital structure after enslaving the human race) explodes. Throughout Episode 2, the player witnesses the after shocks of the portal storm until they are are stopped towards the end of Episode 2 where the player launches a rocket from White Forest base and detonates it near the portal, neutralizing it completely.