In Lithuania and Latvia, border-crossing tunnels were discovered only after they were already completed and in use. Illegal border crossers are not just refugees.
Tunnels can be used to transport contraband and illegal border crossers. Veiko Kommusaar, Deputy Director General of the PPA, said that tunnels could be detected during excavation.
There are physical measures, technical solutions, and seismic sensors that detect such activity. Colonel Ants Kiviselg, head of the Defense Forces Intelligence Center, spoke about this.
Estonia is covered with seismic stations. For example, the Piusa Caves have a seismic station that measures ground vibrations. Heidi Elisabet Soosalu, a seismologist at the Geological Survey, said that the Piusa station would detect excavation if it occurred near the station.
Soosalu said that a better option would be to use fiber optic cables, known as distributed acoustic sensing. This system monitors ground vibrations and sends laser signals back and forth.
Distributed acoustic sensing is a new technology. The PPA does not yet have sensors, but there are companies in Estonia that have them. Veiko Kommusaar said that Estonia's neighbors also have the necessary equipment.
The head of the Border Guard confirmed that sending soldiers through a tunnel today is unlikely. The Defense Forces confirmed that the assessment of conventional threats has not changed.