Variations in citations and impact factor of papers focused on subterranean systems over time. a Average ± standard error number of Web of Science citations to articles. Citation counts are normalized by the year of publication b Average ± standard error Impact Factor of the journals in which the papers are published. Graphs are based on a database of 21,486 research articles focused on caves and other subterranean habitats published between 1991 and 2018 (Martínez and Mammola 2020). The yearly Impact factor of journals is based on the periodical reports by the Journal Citation Reports of Clarivate. For this graph, the year 1997 was chosen as a starting point because around this date the use of Impact Factor started to become widespread (Archambault and Larivière 2009). Note also that the overall Impact Factor of scientific journals increased over the last 21 years, a feature that may have partly inflated this trend.