The tunnel was found Tuesday, officials said. Officials said in some places the tunnel was more than 60 feet deep and described it as hundreds of feet long, extending from Gaza to southern Israel, and lined with cement.
Israel blamed Hamas, which has controlled Gaza for years, for the tunnel.
Security sources said that tools and other evidence found on the site suggested the tunnel's makers were interrupted while they worked on it. The sources said it is the biggest one the Israeli military has uncovered, the Post said.
"We understand the tunnel's exposure is a blow to Hamas," a source told the Post. "Tunnels are an ethos for Hamas, and the source of many of its operational successes. As soon as it loses three tunnels in a number of months, that represents a major blow to it."