Yes they might be. We can only see a few meters into them from the surface, so we don’t know how far the caves extend, especially as the regions near the pits are probably partly filled in with debris as well. But they could be massive caves.

Some of them could be large enough to fit an entire city within, in the low lunar gravity. I’d say the top two future lunar base locations are the lava tube caves, and the “peaks of eternal light” at the poles.

So far we can only see cave entrances, but the extensive systems of rills and the Grail data are suggestive of larger caves to discovered. Some of the possible lava tube gravitational signatures are over 100 kilometers long and several kilometers wide. Yhat's similar in size to the O'Neil cylinder space habitat with a land area of several hundred square miles (the O'Neil cylinder consists of a pair of cylinders, each 20 miles long and 4 miles in diameter, with total land area 500 square miles).

Each such cave could house several million people. This may be a long shot, but isn't it amazing, to think that the Moon could have caves as vast as this, similar in size to an O'Neil cylinder, and we simply wouldn't know yet?

EXAMPLE LUNAR CAVE SKYLIGHTS - LACUS MORTIS, MARIUS PIT AND THE KING-Y NATURAL BRIDGE

The Lacus Mortis area has possible volcanic cinder cones, as well as the more common shield volcano features, rilles, and a partially collapsed cave entrance with a gentle slope leading into it. This is the destination for the Astrobiotics mission in 2014.

Partially collapsed "skylight" in the Lacus Mortis region of the Moon.

Photos of the Lacus Mortis pit from various angles, which were used to build a 3D model of the pit, assuming that it is a cave entrance.

Another interesting pit is the Marius Hills pit entrance, original destination for astrobiotics:

This shows the topography - it's about 40 meters deep The crispness of the landform suggests the collapse happened less than a billion years ago, and the lack of any raised rim or eject suggests it formed through collapse, not through a meteorite impact.

This image shows an oblique view. It's viewed from an angle of 45 degrees, and the light from the sun is at an angle of 34 degrees from the vertical. As a result they were able to confirm that the area of the floor illuminated in this image continues at least twelve meters under the overhang. Papers here, and here .

This shows the location of the Marius pit along a lunar rille. Image from page 5 of Exploration of Planetary Skylights and Tunnels

Another "honorable mention" goes to the region of King crater, which is of special interest for its remarkable natural bridge.

Lunar natural bridge feature King Y, probably caused by a double collapse. It's about 7 meters in width and a 20 meters walk to cross it.

The lunar caves may also have unusual minerals that formed as the lava that created the cave slowly cooled and differentiated.

The NASA PERISCOPE project, currently a phase II concept study, could potentially give us a way to see into lunar caves from orbit using femtosecond laser photography which lets you "see around corners" to parts of the cave that were never within the line of sight of the orbiter.

We may may get our first views into the interior of a lunar cave from ground level some time in 2017, with the Japanese Hayuto Lunar X prize contender Moonraker, which will explore the Lacus Mortis pit "skylight" and then lower its two wheeled rover Tetris into the pit . For details of this mission, see Robotic missions to the Moon, already planned, or near future, from 2017 onwards, below.

See also Lunar caves as a site for a lunar base in my Case for Moon First - the above is a slightly shortened edited version of it.