def _obj_verts(vertex_coords):
    """
    Return a string representing the vertices in OBJ format.

    Parameters
    ----------
    vertex_coords: numpy array of floats
        A 2D array containing 3 coordinates for each vertex. Dimension is (n, 3) for n vertices.

    Returns
    -------
    string
        The OBJ format string for the vertices.
    """
    if vertex_coords.shape[0] == 0:
        return ''
    vert_reps = ["v %f %f %f" % (v[0], v[1], v[2]) for v in vertex_coords]                      # x, y, z coords
    return '\n'.join(vert_reps) + '\n'