1. A computer-implemented method, comprising: receiving, by a data processing device, data specifying a search query that was submitted by a user; determining, by a data processing device, a particular search topic with which the search query is associated; obtaining a measure of relevance of a topic corpus to the particular search topic; obtaining a topic search volume value for the particular search topic based on a value indicative of a number of received search queries that correspond to the particular search topic; determining, by the data processing device, a topic corpus quality measure for the particular search topic based on the measure of relevance and the topic search volume value; determining, by the data processing device, that the particular search topic with which the search query is associated is classified as an underserved topic based on the topic corpus quality measure being less than a specified threshold value; and providing, by the data processing device and in response to receiving the search query, data that cause presentation of a notification that the search query is associated with an underserved topic.
5. The method of claim 1 , further comprising: obtaining topic corpus quality measures for other search topics that have topic search volumes that are within a threshold value of the topic search volume for the particular search topic; and determining the threshold value based on the topic corpus quality measures for the other search topics.