The Egyptian military has begun flooding tunnels used by Palestinian militants and smugglers under the border with Gaza, reports say.
It is the latest move by Egypt to destroy the tunnels, part of an offensive against insurgents.
Excavators have been digging there for weeks to turn the area into what authorities say will be fish farms.
Egypt has destroyed thousands of homes and evicted families from the border, where it is creating a buffer zone.
Hundreds of tunnels were operating until Egypt began to eliminate them in recent years, but some remain functional and others have been rebuilt.
The tunnels, which emerge in the Sinai Peninsula, have played a vital role in the economy of Gaza, which has been under a blockade imposed by Israel and Egypt in 2007 as a measure against the territory's Islamist Hamas rulers.
Hamas has accused Egypt of collaborating with Israel to try to further isolate Gaza.
Scores of Egyptian soldiers and civilians have been killed in an insurgency which has intensified, especially in the Sinai, since the overthrow of Egypt's Islamist President Mohammed Morsi in 2013.