Seismologist says border tunnels could be detected during digging

Tunnels dug under the borders of Lithuania and Latvia were discovered only after they had been used, but monitoring systems could detect excavation earlier, a seismologist told ERR.

Such tunnels can be used to move smuggled goods and people across borders. They could also, in some circumstances, be used to send military personnel into a neighbouring country, said Veiko Kommusaar, deputy director general of the Estonian Police and Border Guard Board.

Colonel Ants Kiviselg, head of the Estonian Defence Forces Intelligence Centre, said border controls could be strengthened with physical measures and technical systems, including seismic sensors that detect underground activity.

Estonia has about 12 seismic stations monitoring natural activity. One is in the Piusa caves in southeast Estonia. Seismologist Heidi Elisabet Soosalu of the Estonian Geological Survey said it could detect underground digging near the station. It has also recorded seismic activity from explosions at an ammunition depot in Toropets, Russia, in autumn 2024.

Soosalu said fibre-optic cables could provide more effective monitoring along the border through a technology known as distributed acoustic sensing. Laser signals sent through a cable can detect small changes caused by ground vibrations, as the cable stretches and compresses.

The Police and Border Guard Board does not currently have these sensors, Kommusaar said, but companies in Estonia and neighbouring countries do. Poland also helped Lithuania detect tunnels, he said.

Kommusaar said it was unlikely soldiers would be sent through a tunnel. The Estonian Defence Forces said its assessment of conventional threats had not changed.