These Seattle oil drums remind me of the packing of circles in the plane. A family of touching sets in the plane is said to form a packing if no two of the sets overlap. It is of practical interest to find the packings which are the most dense. The best packings of congruent triangles, or square, or regular hexagons are intuitively obvious. The best packing of regular pentagons poses more of a problem. The best packing of congruent circle seems intuitive too: just place the centres of the circles at the vertices of a (regular) hexagonal grid. Surprisingly this was not proved until 1940. Many years ago I worked on finding the best packings of spheres in 7 dimensions: not very practical!
