For instance, let's have a look to a picture from an article they have published in 'communications of the ACM' in June 2012 by Dennis Abts and Bob Felderman, titled "a guided tour of datacenter networking".
On this figure, we see that the datacenters are made from server piled in racks. On the top of the rack (ToR), a switch collects the network cables and guarantees local connectivity. A local network, made of switches (probably Ethernet) guarantees intra-datacenter local network connectivity. Then, IP comes into the dance, because the clusters needs to discuss with one another, the datacenter too, and connectivity to the Internet is required to provide services to the end user.
Another paper, titled "What Devices do Data Centers Need?" by Cedric F. Lam, Hong Liu, and Ryohei provides a complementary figure, reproduced below:
On this figure, we see better the interdatacenter part: it relies on point to point optical links with high capacity. As we have seen before, the datacenter routers are interconnected to guarantee the connectivity at layer 3. This is visible in the blue layer of the figure above, with the IP / MPLS backbone (MPLS is my fair guess, because they need speed and quality of service). Finally, some routers of the backbone act as Autonomous System Border Routers and interconnect with Google BGP peers to connect Google's networks to the Internet.