A Chinese "smart" research vessel has pushed deeper into waters off southern Taiwan, a bad sign for Taiwan's defenses and another step up in Beijing's gray-zone pressure.

• The 2,000-ton Tongji passed through the Penghu area early Thursday and was tracked to waters west of Xiaoliuqiu.
• The spot sits near the mouth of the Gaoping River and the approach to the Zuoying naval base — complex underwater terrain that would suit submarine ambushes.
• Officially it does ocean science, but it is suspected of dropping instruments to collect salinity and temperature data useful for submarine and anti-submarine warfare.
• Taiwan's coast guard shadowed the ship the whole way to stop it crossing the line or carrying out illegal surveying.
• The same vessel clashed by radio with Taiwanese coast guard crews off the east coast in May and has repeatedly turned up near Eluanbi, Hualien and Xiaoliuqiu.

Outlook: Expect more of these voyages and more coast guard shadowing, with Taiwan treating the surveys as military groundwork rather than science.