Do BBLs sag over time?

Excuse me, could you please elaborate on the phenomenon of BBLs sagging over time? I'm quite intrigued by this topic, and I'm wondering if there's any scientific basis or empirical evidence to support this claim. Could you also explain what BBLs stand for in this context? Are they a type of investment, a specific technology, or perhaps something else entirely? I'd appreciate any insights or clarifications you could provide.