draw the depth value in opengl using shaders

I want to draw the depth buffer in the fragment shader, I do this:

Vertex shader:

varying vec4 position_;

gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
position_ = gl_ModelViewProjectionMatrix * gl_Vertex;

Fragment shader:

float depth = ((position_.z / position_.w) + 1.0) * 0.5;

gl_FragColor = vec4(depth, depth, depth, 1.0);

But all I print is white, what am I doing wrong?