We present results related to satisfying shortest path queries on a planar graph stored in external memory. In particular, we show how to store rooted trees in external memory so that bottom-up paths can be traversed I/O-efficiently, and we present I/O-efficient algorithms for tri-angulating planar graphs and computing small separators of such graphs. Using these techniques, we can construct a data structure that allows for answering shortest path queries on a planar graph I/O-efficiently.