13. A memory device encoded with a computer program comprising instructions that when executed cause one or more computers to perform operations comprising: receiving data specifying a search query that was submitted by a user; determining 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 a data processing apparatus, a topic corpus quality measure for the particular search topic based on the measure of relevance and the topic search volume value; determining 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, in response to receiving the search query, data that cause presentation of a notification that the search query is associated with an underserved topic.
13. A memory device encoded with a computer program comprising instructions that when executed cause one or more computers to perform operations comprising: receiving data specifying a search query that was submitted by a user; determining 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 a data processing apparatus, a topic corpus quality measure for the particular search topic based on the measure of relevance and the topic search volume value; determining 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, in response to receiving the search query, data that cause presentation of a notification that the search query is associated with an underserved topic. 17. The memory device of claim 13 , further comprising instructions that cause the one or more computers to perform operations 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.