According to google it's happened before. The highest runs scored on day 1 of a test was 494 in 1910 for Australia against South Africa. So we'll see what happens in that regard.
A/ Is the pitch an utter batting paradise?
B/ Are the bowlers having an awful day?
C/ Is everything coming off for the batsmen in a way that is just freakish?
D/ A bit of all of the above?
A/ Is the pitch an utter batting paradise? YES
B/ Are the bowlers having an awful day? YES
C/ Is everything coming off for the batsmen in a way that is just freakish? NO The batting has been amazing - well until Stokes went mental in the last 20 minutes
D/ A bit of all of the above?
The fastest a test team has ever reached 500 apparently. Imagine if they declare and Root and Stokes are the only people who have batted, and NOT reached 100
A/ Is the pitch an utter batting paradise?
B/ Are the bowlers having an awful day?
C/ Is everything coming off for the batsmen in a way that is just freakish?
D/ A bit of all of the above?
Lucky for Pakistan that they went off 15 overs early due to bad light, otherwise they might have declared and had a few overs at the opposition at the death.