C char array has different length than expected

I have a very simple code:

secret[]="abcdefgh";//this is declared in different function and is random word
int len=strlen(secret);//ofc it is 8 in this case
char word[len];
for(int a=0;a<len;a++){//expecting this will put '_' at positions 0-7
    word[a]='_';
}
printf("%d %s",(int)strlen(word),word);

However, strlen(word) returns 11 and word contains "________� @", thus there is some obvious memory leak I can't see. Any idea?