So, I send a Development Test of my in-dev game to some friends, and they found out that the Depth Test in OpenGL does not work on Nvidia.

I use my own matrices and sent them to the shader, and at the start of evey game loop I use

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT ); // clear the display

You can see mountains through other mountains and stuff. On my Radeon HD 6650M it works perfectly fine. Any ideas?

I don't have anything special in the shaders, just some basic lighting calculations. I dont touch the gl_FragDepth.

Screen: (Yes, the textures are just placeholders :))