The potential caves were spotted near a massive Martian volcano, Arisa Mons. Their openings range from about 330 to 820 feet (100 to 250 meters) wide, and one of them, Dena, is thought to extend nearly 430 feet (130 meters) beneath the planet’s surface.
The researchers hope the discovery will lead to more focused spelunking on Mars. –Possible New Mars Caves Targets in Search for Life (Space.com)
No word on whether the entrance to the caves is blocked by a 3×3 steel grate.