I had to look it up when my wife asked. I recalled we'd been nearby when staying in Karuizawa many years ago.

wiki
in the news (eruption)

Surprised to see jmnedict contains very few mountain 
names:

http://www.edrdg.org/jmdictdb/cgi-bin/srchres.py?
svc=jmdict&s1=1&y1=4&t1=%E5%B1%B1&s2=1&y2=2&t2=Mount&s3=
1&y3=1&t3=&idtyp=seq&idval=&search=Search&src=2&stat=2&n
fcmp=%3C%3D&nfval=&gacmp=%3E%3D&gaval=&smtr=&smtrm=0&ts0
=&ts1=&mt=0&grp=

Only 14 entries, and only 4 of them concern Japanese 
mountains!