1. (verb) hollow out as if making a cave or opening

• undermine

2. (verb) explore natural caves

• spelunk

1. (noun) a geological formation consisting of an underground enclosure with access from the surface of the ground or from the sea

• cavern
• cove
• grot
• grotto

2. (verb) hollow out as if making a cave or opening

• sap

1. (noun) a geological formation consisting of an underground enclosure with access from the surface of the ground or from the sea

• formation
• geological formation

2. (verb) hollow out as if making a cave or opening

• core out
• hollow
• hollow out

3. (verb) explore natural caves

• explore

1. (noun) a geological formation consisting of an underground enclosure with access from the surface of the ground or from the sea

• spelunk

2. (verb) explore natural caves

• potholer
• spelaeologist
• speleologist
• spelunker