bubble

A spherical pocket of air or gas within a liquid or solid material.

"Children love blowing soap bubbles."

To form or rise as bubbles, or to make a sound like this.

"The water began to bubble gently."

A comfortable or protected situation, often isolated from external reality.

"She lived in her own little bubble."

A period of intense, often unsustainable, speculation or activity in a market.

"The housing bubble eventually burst."