disappearing streams
a karst
Obsidian is usually found in caves underground where flowing water hits still lava.
Tautology. that's what it means. Caves are underground by definition!
Caves tend to be underground, you know. They are shy.... Yes there are. Try "caves in Switzerland" in Google or equivalent.
The Wookey Hole Caves were formed about 400 million years ago by the rainwater boring through the limestone. The underground streams and lakes, which swirled around to form caverns, finally emerge as the river Axe. The caves contain the deepest sump in Britain at 67m.
On the floor! E.g. in hollows where dripping water can accumulate. Often in streams as in surface streams. Or as "gour pools" (aka in USA, I think, "rimstone pools") - formed by calcite deposits forming natural weirs in the trickle of water flowing through them. These can be very beautiful.
ALL caves are underground by definition. It's where they are! :-)
the moon is not like earth. so there are not any underground caves. it a solid!! duh. Updated answer Since the moon is considered to be part of the Earth and there are caves on Earth. Why cant there be caves on the moon.
Animals that: Live entirely underground - Troglobites. Use caves at least partly - Troglophiles (e.g. bats) are in caves by accident - Trogloxenes (they don't thrive, and indeed may soon die.)( Aquatic animals found in caves' streams and pools may be described as above, but with the suffix "troglo" replaced to be more specific by "Stygo".
Primarily by diverting any surface streams and run-off water they intercept, underground. Otherwise, none.... unless a passage roof collapses and forms a doline.
In rocks or in underground caves
Caves are typically formed through various geological processes, such as the dissolution of limestone by acidic groundwater, erosion by flowing water, or volcanic activity. Over time, these processes carve out underground passages and chambers, creating the intricate cave systems we see today.