Event Horizon
https://en.wikipedia.org/wiki/Event_horizon#Event_horizon_of_a_black_hole
https://www.physicsoftheuniverse.com/topics_blackholes_event.html

Spaghettification
https://en.wikipedia.org/wiki/Spaghettification
https://curiosity.com/topics/black-holes-might-cause-spaghettification/

Gravitational Singularity
https://www.space.com/34281-do-black-holes-die.html
https://en.wikipedia.org/wiki/Gravitational_singularity

The Firewall
https://www.scientificamerican.com/article/black-hole-firewalls-confound-theoretical-physicists/
https://owlcation.com/stem/What-is-the-Firewall-Paradox

Other Dimnensions
http://www.hawking.org.uk/into-a-black-hole.html
https://www.sciencealert.com/a-five-dimensional-black-hole-could-break-general-relativity-say-physicists

Nothing
http://www.pbs.org/wgbh/nova/blogs/physics/2015/12/are-singularities-real/
https://www.space.com/38091-the-existence-of-black-holes.html

Gravstar
https://www.huffingtonpost.com/entry/what-if-black-holes-werent-holes-after-all_us_58ae6ebfe4b0d0d07e7c678b
https://en.wikipedia.org/wiki/Gravastar

Closed Timelike Curves
https://www.thoughtco.com/closed-timelike-curve-2699127
https://en.wikipedia.org/wiki/World_line

Wormholes
https://www.space.com/20881-wormholes.html
https://www.physicsoftheuniverse.com/topics_blackholes.html

Other Universes
http://www.nationalgeographic.co.uk/space/2017/10/are-we-living-black-hole
https://www.insidescience.org/news/every-black-hole-contains-new-universe