US-100 Ultrasound sensor doesn't work on Arduino mega 2560
Have you removed the "Serial Data Mode" shunt to put the device in "Pulse Width Mode"?

On the other hand, perhaps the "Serial Data Mode" would actually work better for your application?

Where I work, we have a coding style that strongly encourages
all "magic numbers" to be documented
with a link to the website it came from
or a reference to the book and page it came from.

Would something like this work for you?:

void loop(){
    // Trigger US-100 to start measurement
    // at least 50 us high, then set it low,
    // as recommended by
    // https://www.bananarobotics.com/shop/US-100-Ultrasonic-Distance-Sensor-Module
    digitalWrite(trigger,HIGH);
    delayMicroseconds(50);
    digitalWrite(trigger,LOW);
    // Acquire
    unsigned long echo_microseconds = pulseIn(echo,HIGH);
    // debugging
    serial.println( echo_microseconds );
    Serial.println(F(" microseconds"));
    // conversion factor from
    // https://www.bananarobotics.com/shop/US-100-Ultrasonic-Distance-Sensor-Module
    unsigned long distance_mm = echo_microseconds * 34 / 100 / 2;
    Serial.print(distance_mm);
    Serial.println(F(" mm"));
    delay(50);    
}


It might be easier to use the Ping library as described at
Arduino-Info: UltraSonic Distance Measurement.

You might find the
Arduino Forum: Measuring Distances with Ultrasonic Sensors
thread educational.