An airfoil comprising a body including leading and trailing edges (30, 32) joined by spaced apart pressure and suction sides (34, 36) to provide an exterior airfoil surface, and a cooling passage arranged interiorly of the exterior airfoil surface, the cooling passage being defined by a wall and an opposing wall, wherein the wall includes an interior cooling surface exposed to a cooling flow passing through the cooling passage, the interior cooling surface being spaced apart from an opposite wall by a distance to provide a cavity between the wall and the opposite wall, wherein the interior cooling surface includes micro-bumps protruding from the interior cooling surface into the cooling passage, the micro-bumps discrete from one another in multiple directions along the interior cooling surface and noncontiguous from one another about 360° of each micro-bump along the interior cooling surface, wherein the interior cooling surface includes first and second micro-bumps adjacent to one another and separated by a length in a stream-wise direction, and the first and second micro-bumps are separated in a span-wise direction perpendicular to the stream-wise direction by a width, wherein the stream-wise direction is the direction of the cooling flow passing through the cooling passage when the airfoil is in use, wherein: each of the first and second micro-bumps includes a length, a width and a height and a generally spherical cap, wherein the height is less than the distance such that the micro-bumps are spaced from the opposite wall; and the micro-bump length in the stream-wise direction and the micro-bump width in the span-wise direction are the same and correspond to a radius providing a circular base to the micro-bump on the interior cooling surface; characterised in that: the first micro-bumps and adjacent second micro-bumps are offset relative to one another by a stream-wise offset and by a span-wise offset on the same interior cooling surface; the offset length separating the first and second micro-bumps in the stream-wise direction is about 1 to 10 times the micro-bump length; and the offset width separating the first and second micro-bumps in the span-wise direction is about 0.5 to 5 times the micro-bump width. wherein the interior cooling surface includes micro-bumps protruding from the interior cooling surface into the cooling passage, the micro-bumps discrete from one another in multiple directions along the interior cooling surface and noncontiguous from one another about 360° of each micro-bump along the interior cooling surface, wherein the interior cooling surface includes first and second micro-bumps adjacent to one another and separated by a length in a stream-wise direction, and the first and second micro-bumps are separated in a span-wise direction perpendicular to the stream-wise direction by a width, wherein the stream-wise direction is the direction of the cooling flow passing through the cooling passage when the airfoil is in use, wherein: each of the first and second micro-bumps includes a length, a width and a height and a generally spherical cap, wherein the height is less than the distance such that the micro-bumps are spaced from the opposite wall; and the micro-bump length in the stream-wise direction and the micro-bump width in the span-wise direction are the same and correspond to a radius providing a circular base to the micro-bump on the interior cooling surface; characterised in that: the first micro-bumps and adjacent second micro-bumps are offset relative to one another by a stream-wise offset and by a span-wise offset on the same interior cooling surface; the offset length separating the first and second micro-bumps in the stream-wise direction is about 1 to 10 times the micro-bump length; and the offset width separating the first and second micro-bumps in the span-wise direction is about 0.5 to 5 times the micro-bump width. each of the first and second micro-bumps includes a length, a width and a height and a generally spherical cap, wherein the height is less than the distance such that the micro-bumps are spaced from the opposite wall; and the micro-bump length in the stream-wise direction and the micro-bump width in the span-wise direction are the same and correspond to a radius providing a circular base to the micro-bump on the interior cooling surface; characterised in that: the first micro-bumps and adjacent second micro-bumps are offset relative to one another by a stream-wise offset and by a span-wise offset on the same interior cooling surface; the offset length separating the first and second micro-bumps in the stream-wise direction is about 1 to 10 times the micro-bump length; and the offset width separating the first and second micro-bumps in the span-wise direction is about 0.5 to 5 times the micro-bump width. the first micro-bumps and adjacent second micro-bumps are offset relative to one another by a stream-wise offset and by a span-wise offset on the same interior cooling surface; the offset length separating the first and second micro-bumps in the stream-wise direction is about 1 to 10 times the micro-bump length; and the offset width separating the first and second micro-bumps in the span-wise direction is about 0.5 to 5 times the micro-bump width.