Breaking Space news! I just heard a short item on the BBC news broadcast (ok, podcast). It said that a JAXA lunar orbiter has determined that a previously identified cave is quite extensive - perhaps 50 km long!
What spacecraft could they be talking about, and how does a spacecraft determine the length of sub-surface caves?
So far I've only found holes. See for example LRO Takes a Closer Look at Moon Caves and also Russia Eyes Caves on Moon for Setting Up a Lunar Base where dimensions of 50 to 400 meters (not kilometers) are mentioned.
edit: I have found a ~50 km collapsed cave identified by surface features in this NASA LRO page, but I think the breaking news is that a new, different, not-collapsed sub-surface cave has been identified.
below: Previously identified collapsed cave from LRO imagery, captioned:
This section of WAC frame M117773324 shows the area where the feature transitions from a chain of collapse pits to a continuous uncollapsed segment. A large depression at the northern tip of the chain maybe a possible source region for the flow of lava across this region. The chain is approximately 50 km long. Image resolution 58.9 m/pixel [NASA/GSFC/Arizona State University].