Maybe someone can share some light on this? Why were there HECU marines in Xen? I've read that it was suppose to be Earth, but Valve couldn't have more than one skybox since it was Goldsource in the OG. However, in the Half-life PS2 port (bottom one), Gearbox changed the skybox texture to black to fix this issue I'm guessing. Anyways, I thought I could get some random answers since I just beat the PS2 Port and this thought of mine came back again. Originally, I like to have thought some of the HECU Marines invaded Xen or they just got teleported there... or maybe they aren't actually there.