Which Colliery has a Death toll of 7?
CREATE TABLE "mining_disasters_in_the_rhondda_valley_1" (
  "colliery" text,
  "location" text,
  "date" text,
  "year" real,
  "death_toll" real,
  "cause" text
);
SELECT "colliery" FROM "mining_disasters_in_the_rhondda_valley_1" WHERE "death_toll"=7;