  Searching for a line on the plane with $n$ unit speed robots is a classic
online problem that dates back to the 50's, and for which competitive ratio
upper bounds are known for every $n\geq 1$. In this work we improve the best
lower bound known for $n=2$ robots from 1.5993 to 3. Moreover we prove that the
competitive ratio is at least $\sqrt{3}$ for $n=3$ robots, and at least
$1/\cos(\pi/n)$ for $n\geq 4$ robots. Our lower bounds match the best upper
bounds known for $n\geq 4$, hence resolving these cases. To the best of our
knowledge, these are the first lower bounds proven for the cases $n\geq 3$ of
this several decades old problem.
