Unspeakable Tragedy?

According to Boing Boing:

Following up on the understated elegance of the 9/11 lamp, I present to you another lighting fixture of unspeakable tragedy: the 1979 Three Mile Island commemorative lamp.

Thre-Mile Island? Is that where the worst (i.e., nothing) happened? I noticed no details were given about the alleged tragedy …

… or was it called “unspeakable” because no details could be spoken?